Dear pyrotech and ReaperWolf,
Thanks for your interesting answers.
Indeed, one of my player, embodying a rich merchant-thief, wants to start the game reasonably wealthy but being noble does not fit his background.
Hence my asking for a solution.
At firt I've tried a fixed conversion of 1DP for 2 gp, yet it did'nt fit well, as a 20 DP expense would result in a mere +40 GP, which is rather inconsistend with the price of, say, a battle horse, or even irrelevant compared to the Noble background that one can purchase for 20DP (which, in my opinion, may give access to land, proper-sized mansion, serfs & servants and the like).
I may try to go for the solution proposed by Old Man but granting a bonus on the roll, because spending 5DP to roll on the Rich column of the Money Table should end up with a 5 SP gain in case of bad luck.
However, the idea of an independant income talent, combined with the expense of DP as "month of savings" might be a solution as well.
Another might be to purchase extra "starting money roll" by spending DPs (say for instance un extra 10+1D10 for 2 or 4 DP).