It depends on the tyres, and the life left on them, but looking at Sold listings on eBay, only 2 recent examples come up. One set at £627 and one at £1495
Usually go for 200-700 for a set with alloys and low miles, it's kind of a buyers market as they usually (UNLESS MY18 GT!) come with pzeros which nobody wants, and if anyones buying them it's to put winter rubber onto or use as a backup set.
However, as stated before, it's a buyers market, sometimes they sell for £1000, othertimes £200, there's no real "guide price". My best bet would be to slap them on ebay, and let people bid on them and post them on all the mustang groups (Unleashed/Smuk/MOC/MOCGB/Unusual Suspects/NPC - etc)
I paid £600 for 4x unmarked Nickel wheels without TPMs and with Pzeros (avec 4K wear). I thought that was a reasonable price. TPMs are cheap enough to get hold of, but it's a pain to get them installed.