I think the low resistance FET's are quite expensive. last time I bought some they were 6 bucks each for 3 mOhm SMT's. I doubt HL decided to put in an extra $40 in hi performance FET's. They probably have about 50 cents to spend on power fets. Of course, they can order hundreds of thousands of them so they get good price breaks.
To get good heat transfer into the board you should have something with heavier copper than the 1/2 ounce stuff they are using. 2 ounce board would be nice but once again, you pay for it.
It's expensive to make a low cost tank ESC. HL has done a pretty good job.