This is probably harsh to say, but if a $500/mo expense is what breaks the bank, you can't seriously claim to be operating commercially successful software. Even small businesses spend $XX,000 a month on labor and transaction fees and vendors and rent...etc. If the option is $500/mo for some other guy's work or $3,000/mo to hire a new developer to work on recreating the already existing system full-time, the choice is pretty clear.
I'm not doing commercial software for end users. My projects are usually in-house apps for midsize businesses. The largest one is a local hotel chain whose entire server infrastructure I've gotten to under $400/mo. That serves their employee-facing apps (front desk and back-of-house), their reservations system (~1k bookings per day), their website (~100k uniques per month), email systems and training software. If I told them it was going to cost $3k a month, they wouldn't balk. But they've compensated me well for being able to shave it down considerably over time. It's a great company that appreciates owning their own code. If I can write software once that they would otherwise have to pay a subscription for, they'll happily shell out $100k for their own version if it saves them $2k/mo in SaS. Much of what I've saved them over time has gone into my own bank account, and they've ended up with a pretty amazing ecosystem. But also, it has to do with hard lessons I've learned from not relying too much on SaS platforms. So many have come and gone since I started writing software. In the end each one becomes a single point of failure.
Small businesses are definitely not casual about 500$ mo. It is also unlikely you need to rewrite small business shop entirely to move it or that it would take too long time.