Exoneo makes 2 prosthetic feet: the Mahi for moderate to high activity levels and the Upya for low activity.

Both have a spring block under the heel and spring-loaded "metatarsals", which helps push-off when walking:

https://youtu.be/YeZNaM0NaNc

You can even assemble and size them yourself:

https://youtu.be/XjdDyo69JJU

More interestingly, it seems the company has a lot of customers in developing countries, meaning there's a very good chance those feet are affordable. Sadly, I couldn't find prices and I didn't want to request a quote for nothing.

I did find an independent review of the Upya foot (in French here, automagic translation here, accompanying video here), so at least it's not like all the information I could find comes solely from the manufacturer.

The reviewer confirms that the foot was competitively priced: although he doesn't disclose how much he paid for his foot, he does say it's "a prowess considering the price at which it's sold". That bodes well.