Any mk1 is worth whatever someone is willing to pay for it.

If someone is willing to buy a popular for £2000 so be it.

Look at the prices of RS1800s, a mate of mine has a genuine forest arched version that a few years ago was valued at £10000, now one with similar spec (The Irish one) is up for £20000! That is a 100% price increase in less than 5 years!

The prices of all RS models seem to be going up (with the exception of Sapphy Cosworths)

3 door Cossies now making £10k (Which a few years ago 500s were commanding with 3 drs making £6-7k)
RS500s now making £15k+ (A few years ago they were going for £10k)
Mk2 Escort RS1800s (Vary from £10k-20k whereas they used to be £9k-10k)
Mk2 RS2000s are now making upwards of £7k+ (You would've been lucky a few years ago if it made £5k for a minter)
Same as mk1 Mexicos are now making £6-7k+
Even absolute snotters of RS2000s & Mexicos are making close to £1500-2000 whereas a few years ago you could pick them up for anything from £500-1000

S1s are now making £6k+ for minters, you would've been laughed at a few years ago for asking that amount but now people are happy to pay it.

As for modified examples, it all depends on how well the mods have been carried out & how big the spec is.