There are lots of decent ones on EBay for $50 such as
EBay leak down tester
What's nice is all that you need comes in a kit in a plastic storage box. The only difference between a really expensive one and a cheap on for the most part is the quality of the pressure gauges. All the rest of the fittings and the .040" orifice are off the shelf items. but for $50, you couldn't build one yourself any cheaper.
The fundamentals of a leak down tester.-
You need an air supply of greater than 100 PSI. You supply air to the leak down tester and adjust the regulator on the leak down tester to 100 PSI. That air source then goes through a .040" orifice and supplies the fitting that's screwed in to the sparkplug hole and the pressure after the orifice is compared to the supply pressure. Sometimes that gauge is in PSI and sometimes the gauge is represented as a percentage. So on some leak down tester, the down stream gauge might read 94% or it might read 94PSI but it's the same thing. 94PSI/100PSI is 94%.
The assumption is that an engine that leaks less than the amount of air that could pass through a .040" orifice is 100% sealed and anything that leaks more than what could pass through the .040" orifice is leaking as a percentage of a "sealed engine"
For the most part, I only use the leak down tester to "indicate" if I have a problem, then I go checking for where the leak seems to be. Intake valves will leak in to the intake manifold, exhaust will leak in to the exhaust manifold, piston rings leaking can be heard at the oil fill cap and head gaskets usually can be seen as bubbles in the radiator. All I care about when I see the leak down rate, Is if it's a grouse leak or a minor leak and then where I think it is then I decide if I'm going to go after it or record the leak rate as a reference for the next time I do the leak down testing as part of my predictive maintenance program. My time in nuclear has taken me from breakdown maintenance / preventative maintenance to preventative maintenance / predictive maintenance. You won't find me sitting on the side of the road with busted equipment or if you do, please shoot me and put me out of my misery. I'd be too embarrassed to be found broken down like an "HD "rider.