Here's a reason why you should not buy a cheap DMM. The input filter in cheap meters easily gets confused and attenuates the signal, and worse than that, the amount of attenuation is unpredictable so you have no way to know the real measurement!
To demonstrate, I'll use a cheap meter (about $35) and an expensive meter (about $80) to measure the normal AC line.

So far so good, right? That's because they're measuring an almost pure sine wave.
But measure the UPS output and look what happens!

That's because they're measuring a square wave, which confuses the input filter in the cheap meter. The expensive meter has better input filters that are not confused by high frequencies in the signal being measured.
This inaccuracy is confusing at best and dangerous at worst. A cheap meter can easily cost you a lot in the long run!