it depends on the condition of both. an average e30 m3 sells for 18k to 20k in very good shape. an outstanding very low mileage e30 m3 sells for 30k to 40k. a car with a lot of modifications, say a 2.5L engine with evo3 mods, coil over suspension with mileage might sell for 25k ... all of these are very round numbers. if the e30 is an evo 2 or evo 3 (sport evo), my numbers are way off. of course a person in the usa w/ an evo 2 or 3 or cecotto variant is a very rare thing.
as you are aware, 20k on a coupe may or may not buy very much. to me, a similar condition e9 to the 20k e30 m3, is 30k. the best description for both is a very nice condition driver that isn't rusty and doesn't really need anything.
but as you can imagine - the judge of the comparison will have his hands full. start by asking, if you sold your e30 m3, what would you sell it for? if he says 25k+, start asking some serious questions.