We're looking to get desks that fit both developer preferences: sitting and standing.
All the convertible desks I've seen online cost $500 to even $1200.
Are there any low cost options available? The only requirement is that the desk should be easy to convert from standing to sitting, and vice versa.
I'm a little surprised by what you're asking. It sounds like you're hiring developers that you're probably paying anywhere from 30K/year up to $150K/year, yet you aren't willing to spend $500 for a desk that they'll use that will last them several years. (Heck, the desks will probably last longer than half of your employees.) If it's that big of a deal, just drop their pay by $1K and buy the desk for them.
But to answer your question more directly, after doing some research myself, I agree with your assessment. It doesn't look like this type of desk comes much cheaper than that. You'll have to get a bit more creative on how you solve the problem.
There are places that allow you to rent furniture. Long term that will be a worse deal than just buying them outright, but in the near term, it may save you money. Look around on the Internet for what's available near you.
It looks pretty clear to me that buying just a standup desk or just a sitdown desk will be cheaper for you than one that can be either. You might be able to just buy some of each, and let people pick which one they want. Perhaps everyone can have a sitdown desk, and you can put a few standup desks elsewhere that people can take their laptop to and use.
And of course, Nick does have a valid point. Sometimes, you should just make do with whatever you can. If you're so tight on money that you can't afford $500 for a desk, Nick's wood plank and beer crates would definitely do the trick, at least in the short term.