imo as a general rule id say:-
Ph1s - £700 (totally shagged W reg)- £3000 (mint circa 60k mile Y reg in a rare colour)
mk2 172s - £1000 (totally raped Y reg/ 51 reg with no history - cambelt)- £4250 (mildly modified, good condition, FRSH and immaculate 53/04 reg with low miles)
cups - £1500 (Cat c/d shed)- £4500 - (low mile, good service history and mint confition)
182s with packs - £1750 (complete bucket and no history 100k+ miles, auction special) - £5250 (late 55 regger with low miles and FRSH, immaculate and spotless in a good colour (petrol/ liquid add £500-750)
Trophys - £2500 (that 167k one?!?! or cat c/d + £500) - £6000 (mint late 55 reg with low miles and FRSH and all invoices, receipts, spotless)
If a car is modified it depends what people will think, if with 4 pots, cams, spoilers, splitters, alloys, decats, air intakes, stickers, seats and exhausts its as usual personal preference so probably best punting 2nd hand.