We've visited Betty's Restaurant Grill in the Clocktower Plaza off of Rt. 7 in Sterling (same plaza as Burlington Coat Factory and Shoppers Food Warehouse) several times now, sticking primarily to the saltenas and pupusas. This place is a real gem for bargain dining--the spouse and I got out for under $15, including tip, for lunch on our first venture, and that included a mocochinchi (cold cinnamon drink over a dried peach) for me. If I remember correctly, the pupusas are $1.50 each, and the saltenas $2.50 each.
I have little basis for comparison with a standard for the dish, but the saltenas are delicious. The dough is fairly sweet and contains either a beef or chicken stew, depending upon which you choose. The slaw which comes with the pupusas is a bit tastier than average.
At least one of the staffers does speak English and was able to bring me the mocochinchi when I asked her if they had the drink with the dried peach.
There are plenty of more complex dishes, including a rabbit dish, so this is not just a lunch counter by any means, though the decor is quite plain.