I (Carth, 1.2) got myself a protectorate last night.
Gaul would have no ceasefire, but grudgingly accepted protectorate status.
We had been at war for decades, but neither had conquered any provinces.
They were not my main priority, so I just beat back a few attacks on Corduba and kept their fleets off the Med. The Gauls seemed to be doing mostly OK, but the Romans were starting to cause them trouble when they joined my racket
I'm not sure if my treasury was affected in any way, at least initially.
Being a softy and a do-gooder I spent 65K bribing a Julii general who was besieging my new friends in Patavium. Then I used the general to initiate an attack on the smaller Julii stack next to the one I bribed, getting support from the city. I was looking forward to hang around behind Roman lines, trampling velites and routers, but instead i got a CTD (which is very rare), and went to bed.