I'm having it done in two surgeries as well. I find that this varies from surgeon to surgeon. My surgeon also said that risk of a leak go up way more if you do the surgeries back to back. I'm getting my band removed July 13th and then (hopefully) getting my sleeve in early November barring any shenanigans from insurance.
however, I understand your concern about having to pay for two surgeries. I know I wouldn't be able to scrape up all that money. My best advice is to let your doctor know your financial concerns and maybe he will try to work with you in the best way possible. Honestly, he really won't know what he's getting into until he's in your stomach. :\