1) At the beginning of the case, each party proposes how much money they would like to be spent on their side of the case.
2) The Spending Limit is set to the greater proposal. They then must pay the other party the difference between what they proposed and what the other party proposed (to be spent only on legal expenses for this case).
3) Both sides are prohibited sprom spending more than the Spending Limit on legal expenses for this case.
For example, if A and B are going to court, A proposes a $10k Limit and B proposes a $100k Limit; B has the larger number, so the cap is set at $100k, and B must pay for up to $90k legal expenses for A.
Clearly there are some details that must be worked out (eg what if someone wants to raise the limit halfway thru the case? What about money spent on preparation before going to court?) but i think this would be a good start. This proposal achieves substantive equality without sacrificing procedural equality and without preventing either party from spending as much as they want.