Solo dev in Melbourne. Last 12 months I've been building and selling ops software to small businesses on a monthly sub, about a dozen on it now. One of them also runs a small finance company and has asked me to build their back office from scratch.

Rough shape: a portfolio of active customer accounts, each with separate budgets (rego, insurance, fuel, servicing), money coming in monthly from employers, expenses paid out and recorded, monthly statements to each customer, and the whole thing has to reconcile to one bank account every month. All client money, none of it theirs, so a stuff up is serious. 2 or 3 users to start.

Where I've landed:
- Three phases, fixed price each, half up front half on sign off. Around $48k AUD total. A quoting tool as an optional fourth phase later.
- Software records and calculates, never moves money. Humans pay from the bank, system reconciles.
- I keep the IP so I can sell it to other businesses in that industry, they pay less because of that.
- Small yearly fee for hosting and updating the tax tables.

Never priced a custom build before, only ever done subs, so no reference point. Questions:
1. Fixed price per phase with a written sign off each, is that the right shape or do people regret it?
2. Does $48k sound low for ledger grade work where a mistake costs real money?
3. Anyone done the "I keep the IP, you pay less" thing, what did you wish you'd put in writing?

Cheers