I was going to suggest using an old laptop, but: A brief skim of available software tells me there are very sophisticated products out there, actively maintained... And a whole lot of others. It's a rich technical area but that means that to make intelligent decisions will take a bit of study. A geek is needed. <g>
Here is one product list providing a very high level view of the the top of the field for any that might want to cop a look-see.
So small "appliance" type pieces of tech may be the quick and dirty way to get some measurements w/out a lot of study. However, that's not a given. The time spent vs. results with low end stuff may not actually work out well. It's been my repeated experience that good quality (ie. expensive) tools, parts, material are _much_ easier to get set up and working right - once one gets over an initial learning curve. So time/money saved with small, low cost, stand alone monitors may in the end not return value. It looks like a judgment call, a roll of the dice. If you have a dedicated geek, going to a real system looks like a no brainer, but with no geek - then the small stuff may be worth a try.
To be clear: The cost that matters here is _time_, not money. What saves time is ease of use and reliability. The latter particularly suffers with low cost equipment.