I have limited space and want as many keys as I can get in a 45" space. The only factory unit I've found with the right features is Studio Logic's SL73 (73 keys) for $500.
 
Anyone here ever build their own keyboard controller? This could be fun or a complete disaster but cost would also be a factor.

Hmmm, Evan Kale has a load of videos on building things with Arduinos. If seeking simple on/off input, he made one with 4 computer keyboards (https://m.youtube.com/watch?v=NtWRIHYDzWQ)

I thought about doing this at one point, as I've done Arduino MIDI projects in the past and had some spare keyboards lying around.
 
In the end, I decided not to bother as:
1. Removing the existing logic from the PCB was near impossible.
2. Creating a new PCB was not cost effective
 
 
 

If you're talking a simple MIDI controller, more power to DIY.
 
If you're talking building a legitimate 73-key MIDI controller that's superior to what's commercially available, unless you have a machine-shop and the skills to go along with that, it's probably not going to go well.
 

If you like to tinker, have the skills it is probably possible. For me it would be like attempting to hitch hike to Africa and live to tell the story.
If your goal is to actually make music in a studio I would discourage it.

Right. I would only do it if I could customize it to my own wants & needs.