it depends very much on the quality of the work. for example you can get custom front panels ot PCworld near me for 25 quid but the quality is terrible, bits fall of them as they are soldered weakly and everything is very rough and very... not good lol. however if someone was going to go all out and work out where cable tidy hooks need to go on the case, special holes for piping for cooling, intricate designs cut well and finished well then you could be looking into a lot for a decent case. it all depends on whether people think your time is worth their money.