If you're still figuring out whether or not people are willing to pay for what it is you're doing, then just use PayPal. Or mail paper invoices. Seriously.

Why would you EVER invest a ton of time in a billing system when you're just starting out? Billing is a HUGE time sinkhole. That applies to integrating with another provider (Chargify) just as much as it applies to not rolling your own.

Billing is not your product. Billing WILL make or break your product though, so once you've figured out that a reasonable number of people are willing to pay for it, then make sure your billing works and works well.

Also, if you can't afford $39/mo on a SaaS billing provider, then you're not serious about your startup and it's still just a hobby.