I think the Cromwell is fairly strong, and that's alright. But it's too cheap for its performance - 340/110 vs. 350/125 for an Ostheer PIV, while being just as good, if not better.
Take a look at the stats:
http://stat.coh2.hu/squad.php?filename=cromwell_mk4_75mm_squad_mp
http://stat.coh2.hu/squad.php?filename=panzer_iv_squad_mp
The PIV has stronger armor (180/90 vs. 160/80) and better MGs. The Cromwell has a smaller target size (18 vs. 22), faster max speed, acceleration and rotation along with higher moving accuracy (0.75 vs. 0.5) and higher penetration (135/120/105 vs. 120/110/100), as well as lower scatter.
Based on that, I would say the Cromwell is at least as good as the Panzer IV, so it should cost as much. I don't think a slightly increased fuel cost (+10-15) would hurt.
It should not cost more, because the P4 got alot better veterancy bonuses. Hell, the Sherman is a better tank imo and costs 110 fuel also. vet 3 P4 > vet 3 Cromwell