I really like buying shoes on eBay - my groaning shoe rack and even more loudly groaning fiancee will attest to this. A lot of people are quite squeamish about it, particularly if the shoes have already been worn a bit, but if you're shopping on a budget then buying on eBay will open up a whole new world of options that would otherwise be locked away behind their RRP.
Spending £400 in the shops will leave you in the realms of Crockett & Jones - nothing wrong with that, but applied carefully on eBay you can get Edward Green or even Gaziano & Girling in good condition. And provided you don't overpay, you can generally sell them on relatively easily in case of bad fit with minimal loss (or even slight profit!)
If you want to know what sort of makers you should be looking for, have a look at my list of major British shoe brands. I won't be covering all of those brands (and sub-brands) here, but leave a comment if you have any questions on them.
Caveats and notes
Broadly this article will exclude "bargains" - eg. an item that the seller doesn't really know the value of, which you should jump on as ruthlessly and remorselessly as a killer whale on a baby seal. With regular checking of new listings, you can get brands like Edward Green or John Lobb for 20% or even 10% of the RRP. But these crop up intermittently and too unpredictably to usefully discuss or plan for. This is more of an idea of what you can typically expect to pay, plus or minus whatever you may be able to negotiate with the seller.
Also keep in mind that auction-style listings can net good bargains, but it of course depends who exactly you end up bidding against. But It Now style listings may give you more leeway to haggle with the seller on the price, and I personally find the unpredictability of auctions as stressful as they are exhilarating.
I've also generally excluded bespoke shoes from here (from makers like John Lobb St James, G.J. Cleverley or Foster & Sons) as they have their own weird quirks in pricing. Check my article here for more detail.
A quick summary and factors that affect price
You can normally expect to get a pair in as new condition for 40 - 50% of the RRP. In well-used condition, this drops to something like 25 - 30%.
Expect to pay more for the following:
Shoe trees included in listing (particularly lasted shoe trees or branded trees from the same maker)
Full box, bags and any accessories included from the original purchase (some people like myself like to keep as many of the branded boxes as possible, even if it's just to stash in a closet)
Unworn or near unworn condition. Looking at the uppers can be a bit tricky with this - sole shots are the easiest way to tell. Even slight wear on pavement will begin to wear away the sole's finish. You can also inspect the logo on the footbed - heavy wear to the print implies that the shoes have been worn for some time
Exotic leathers like crocodile, alligator, peccary, kudu, elephant, shark etc. will all command a higher price premium (and may even cause import issues depending on your location, so check carefully!)
Rare, vintage models from some brands will have a higher price, as will the more popular models like the Edward Green Dover or Galway
Conversely, you should expect the following to decrease the price:
No box or packaging
Heavily worn, scuffed or even damaged leather (be wary of full rips or tears to leather, especially around the lacing points. This can lead to larger, unfixable tearing very quickly with shell cordovan)
Heavily worn soles - see this post for what to keep an eye out for . Generally, the worst case is a resoling may be needed, but factor that into the price
Really weird bespoke or customised shoes, either because of odd sizing or just plain outlandish design choices
Odd-sized shoes (as in mismatched) or just unusually sized shoes (eg. very small or extremely large). The reality is the market for odd sizes is just smaller, so that should be reflected in price (at least if the seller actually want to sell them)
Gaziano & Girling - RRP £1,300. Expect to pay around £600 - £700 for a new pair - more for exotic leathers, rare or newer models. G&G rarely dip below £500 for a new pair, though well-used pairs can be snapped up for £400 - £500.
Edward Green - RRP £1,200. Expect to pay around £400 - £500 for a new pair. More for iconic models like the Galway or Dover, which tend to hold their value very well. Well-used pairs can be had for as little as £250 - £300.
John Lobb - RRP £1,100. Pricing can be all over the place for John Lobb, and you can get new pairs for £400 - £500. There is a large volume of older and well-used John Lobb shoes on eBay, and prices can get as low as £200. Rarer models like the anniversary editions or those from the Prestige line will tend to command a higher price.
Alfred Sargent - particularly the Handgrade and Exclusive collections. AS are an unusual case - since they recently went out of business there are some frankly incredible bargains to be had (although in a limited range of sizes). RRP £500 - £600. Expect to pay £200 - £300 for a new pair. Used pricing can get very low, right down to £150 - £200.
George Cleverley - RRP £600 for the RTW collections (not Anthony Cleverley or the bespoke brand). Some good bargains here - can be had new for £250 - £300.
Crockett & Jones - RRP £450 for the main range; £600 for the Handgrade range. It's a brand that holds its value relatively well - as new condition for the main range can be hand for £250, while the Handgrade will be around £300.
Tricker's - RRP £450. Tricker's actually have an outlet shop on their own website, and while they can easily be had in new condition on eBay for £250 - £300, used ones can be a real bargain, potentially dropping down to £150.