Anyone with experience in travel and travel APIs wants to share feedback?

Hello IHs!

I built a proof of concept to find flight destinations fitting your schedule.

For example: I live in Lisbon, I want a weekend away, I don't care where but I can fly on Friday between 6pm and 9pm and I want to be back on Sunday between 3pm and 7pm.

My challenge is that I don't find a way to cheaply offer this for all cities worldwide. Full API access would cost around 2,500 USD monthly, which I'm not willing to spend on this fun project right now.

If I don't find a solution, I'll probably abandon the project all together, as I don't see a way of launching this with a limited supply of cities.

So looking for your ideas, anything which might remotely help to make this project viable!