Substitute for FTDI board?
All prices in USD w/ free shipping.

FT232RL breakout board:


Aliexpress: $1.55
eBay: $1.81


They are probably counterfeit. A Windows driver update (AKA "FTDIgate") at one point "bricked" them but I'm not sure that's an issue anymore. Even if it is it's Windows only and easy enough to work around. I've used these for years on Windows and they work fine for me.

If that's still beyond your budget there are a few other common options. I haven't used these breakout boards but I do have quite a few boards that use the CH340G and they work perfectly once you install the driver. There's a decent chance some of these will also be counterfeit. Some didn't break out the DTR/CTS pin for auto reset but if you're on such a low budget maybe it's worth it to you to press the reset button on every upload to save a few cents. They may not have the standard FTDI header pinout and if so would require you to use jumpers to connect them to the Pro Mini rather than plugging directly into the header.

PL2303 breakout board:


Aliexpress: $0.50
eBay: $0.74


These don't appear to break out the DTR or CTS pins so they won't do auto-reset.

CH340G breakout board:


Aliexpress: $0.72
eBay: $0.93


These don't appear to break out the DTR or CTS pins so they won't do auto-reset.

CP2102 breakout board:


Aliexpress $1.00
eBay: $1.31