Re: 308 new gen pmags
There is only one generation of the 20LR/.308 PMAGs. To my knowledge, Magpul has no plans to offer "windowed" versions of these mags, or to offer them in different colors, or to offer them in 25-30rnd capacities, etc. But then again, Magpul is about as tight-lipped on product innovations/changes as they get. If they make any changes or offer additional versions down the road, you'll only find out shortly before they become available to everyone.
As for good prices...anything around $18-19/mag is good. There is (or was) a group buy going on here on the Hide where you could get them for $18/mag. Also, MidwayUSA ($18.95...use a discount code to get them cheaper than that depending on how many you are buying), and Rainier Arms ($18.95), both have them in stock and ready to ship last time I checked.
Someone recently posted that if you have an FFL or a C&R, Brownells sells them for around $14/mag...something else to consider if that applies to you.