converting lots (100+) of analogue audio sources to RTP/UDP/'IP packets'?
You know that shopping questions are not allowed here?  Fortunately, I have never been one to follow the rules... 

Large amounts of audio over Ethernet is not easy-- or cheap.  I've been doing this professionally for the past 14 years, and I still have not gotten the price down to what I would consider cheap.  

I would not recommend a DIY approach to this.  Building the PCB's, writing the software, testing, etc. is difficult for this type of project.  That's fine if you want to start a new career in audio over Ethernet, but this is probably too much for someone who just does this as a hobby.

For commercial products, the cheapest that I know of are the boxes by Atterotech.  They follow the Cobranet protocol standard and so will inter-operate with other Cobranet devices.  But while I said this is the cheapest, it is not cheap!  Also, this is pro-audio gear, with pro-audio performance.   Other companies that make similar products are QSC Audio, Rane, Whirlwind, Peavey, Biamp, and many others.

There is not much for modules that does both the networking interface and the ADC/DAC circuits.  In a former life I designed the Cirrus Logic CM-1 and CM-2 modules which will do up to 32x32 channels of networked audio-- but they do not include the ADC's and DAC's.  Connecting converters to these modules is not difficult, but might still be beyond what you want to do. 

There are other modules similar to the CM-1/2 from Audinate, Lab X, and others.  But I do not think that these will be any easier or cheaper for your uses.