A 10 off fuel cost would mean it still costs 10 more MP than an IHT does...
The bren carrier needs to cost at most 40 MP more and 5 fuel more than a Jeep does(150 MP and 15 fuel) with a popcap cost of 3 - then it would actually be somewhat worth it. No changes to MMG carrier, though ^^.
I actually think this price balance is pretty on the money, I don't see anyone field these, and if I did, I'd call them crazy!