I bought one of those boards asian boards, I think it is generically called a winnwt? or a nwt500? It does a decent job for the money from 0-500Mhz. It can also be used as a simple SA as well as a DDS signal source. Gives impedance with phase, has some built in attenuators, etc. The RLB that came with it works ok though I built my own directional couplers using Minicircuits transformers that seem to work better. The NWT500, just search on ebay, only runs like $175 but it maps perfectly against my SA with TG.
If you just want scalar output, all you need is a noise source and a scope with FFT and long persistence. That will give you a curve but you have to work to get impedance and no phase.
I am also playing around with the ADL5519 from Analog devices. A pair of them with I/Q signal source will give you forward/reflected as well as phase though I haven't worked out the phase part. Those chips aren't cheap, but with a 20db directional coupler you can also use it as a fairly accurate power meter across 60db or so without switching. The app note and data sheet have good examples.
So if you had a pair of the ADL5519 for like $9 per = 18, plus an I/Q DDS for $50 plus two DCs with connectors at $20 per, etc and a microprocessor, you would throw together a simple VNA with power measurement for under $200. I haven't done it yet, but plan to and I will add external ADC 16 bit or more to get higher resolution than the typical micro at 12bit with averaging.
I've been looking at VNAs and commercial high-function products are very expensive. But using the NWT500 coupled with a computer should give you just about everything you would want. You would have to write software to do in channel power measurement and other things my spectrum analyzer does but hey, it's a project.
Jerry