Best diff you can get for that money is a 7075 cup with aluminum housing. But depending on the amount of cells, the 7XL is quite some stress on the drivetrain.
I ran my 7XL on my lightweight maxx with great succes on 7075 maxximizer cups with integy houses. This is the cheapest diff upgrade you can get.
Another alternative is to get the FLM cups with the new 3.3 diffcase. (but this is a compromise imo) 40 bucks is unfortunately not enough to beef up your diffs to be durable as heck.