If your product doesn't make you hundreds of dollars, you'll likely end up losing money.
Is it still possible to promote products under $20?
1) The A in front of GPL is the kind of advertisement that you always want to see.
2) It's hard to argue with a free product, so sales will be high!
3) Forget just ads as loss leaders. Do it to your entire code base!
4) You'll be hailed as a maniacal libre genius!
5) VC bosses will universally love you! Your career will flourish!
Joking aside, the market is open for another generation of authentic open source companies built on good free software, organic growth, and community based advertisement. It might be too extreme to apply to OP's situation, but anyone that needs to hear it should consider it as a powerful and extreme strategy!
Look into all the crypto bro scams, pyramid schemes, and grey market.
The world of advertising can be devoid of morals at one end or throughout.
I think the best movies have the least advertising and stand on their merits as well as word of mouth.