To my knowledge, nothing over 20 is available for the DPMS pattern mags. And they are still at least $20/ea. (P-mag; the OEM mags are $45)
The Magpul .308 Pmags are $18-19 if you shop around. But I have two and haven't been as impressed with them as with their .223 Pmags. The C-Products DPMS .308 type mags are as low as $15 at some places and are, IMHO, pretty good. Neither those nor the DPMS OEM mags impress me, however. I have read that Knight's SR-25 mags are great, but I have not personally seen or used any of them.
On the Armalite side, if you can afford a $1300 rifle shooting ammo that costs upwards of 50 cents/round, you can afford a couple of $30 mags. It was only a couple years ago that DPMS mags cost $35-45 each and few people were complaining about their pricey, proprietary mags. And as noted, if you're really really cheap you can make AR-10 mags by converting M14 mags.
I have no problem with the DPMS rifles, but I love Armalite for their quality and their CS (never had a rifle problem, but very helpful when ordering accessories or just asking questions).
I own a DPMS LR-308B and an Armalite M15A4 (.223) and find the Armalite nicer in just about all ways. I admire the DPMS for its price point and accuracy, and price is why I bought one. And I have shot a 1/2" group at 50 yards with it, which is not bad (I have only maybe 50 rounds total through it; please note, this is at 50 yards, not 100, so it would equate to around 1 MOA, which is consistent with what many people report for the DPMS models with a free float barrel - and I haven't shot it enough yet to know if this is repeatable or one lucky group). But on the whole I am not impressed with it, and generally wish I had either not bought it at all or had instead ponied up the extra $300 for an Armalite AR-10, which I had been wanting before and find myself still wanting since buying the DPMS. This is just opinion, of course - I haven't shot an Armalite AR-10, just admired the ones I handled and looked at.
For fear of killing a few more sacred cows and making myself really unpopular, I have concluded that for my personal uses, a good bolt-action rifle offers better accuracy than any of the .308 ARs for less money than even the DPMS, and an FAL or M1A offers most of the accuracy I can use with reliability, solid engineering and a solid military track record, although admittedly not quite the level of practical accuracy* that is possible with a top notch .308 AR platform. This is just my personal conclusion, and I'm not telling other people what to do.
(*The high-end match M1As can offer 1 MOA or better, but require serious original work and maintenance to get there and stay there. A really tricked out FAL with a free-float forend can also get in that ballpark, typically with single loading of the mag, but loses its practicality if you go that route. The nicer .308 ARs have the potential to offer reliable semiauto operation with MOA accuracy, but cost plenty to get there.)