I build API systems daily for myself + clients.

Also other on-demand non-API systems.

To provide you an answer requires a starting point + gathering a good bit of context.

Best if you book calls with me + other people who build this type of software daily.

Tip: Here's how I approach this for myself + clients.

Whatever code I develop, evolves through stages, which self fund the project.

In other words, every month of development must produce income the next month greater than last month's dev cost.

So if month one's estimate is $10K of dev work, this work only starts when there's a clear path to the $10K of work generating $20K of income next month.

My suggestion.

If you can't come up with a clear plan to have income grow fast enough to immediately cover all development cost, get a new idea.

Hint: Most projects I do for myself + clients at this point use a pre sale model.

So, someone (client, myself, many people) market the product at a discount for a small number of users to be in the alpha + beta stages of release.

Sometimes this can generate... well... substantial income...

This also gives a massive window of debugging, where development + debugging costs are covered.

Think of this as a Kickstarter project where money comes in, sometimes for many months or a year at a discount.

So if your service will run $100/month, pre sell a $1200 package for 2-3 years, rather than the normal year of coverage.

To do this you must be clear with participants...

1) There will be problems to wring out of code.

2) You must be clear you can code some sort of product in a few weeks, to provide some sort of deliverable.

3) Don't spend all the money, so you can do instant refunds for participants who loose patience with alpha/beta code quality.

This approach works super well.