I'm in the same situation, I'm looking for a convertible under $100 that will allow rfing as long as possible. There's not many choices, even if you consider the BRU trade in, as their prices are typically higher than say, albeebaby.com or walmart. Maybe she could sell their seats to make up some of the money? What seats are they currently in? can the 2yo's current seat be passed down to the baby?
As far as seats in this price range, so far, for me, I'm liking the safety1st onside air (rf to 40lbs, $60 on albee or walmart) or for a little more the guide 65 (walmart at $78 rearfaces to 40lbs, then ff to 65lb, cost is a little more than she's looking to spend, but it may be worth it for the 2 year old as she won't need to replace this seat as quickly). Another good choice is the cosco apt, 40lb and 40" rf, and allows rf to top of head is even with shell costs about $50. There's also the scenera, which only rfs to 35lbs, but it only costs $39 at walmart.
there are other seats available in this price range like the evenflo tribute, or the graco comfortsport, but they are often outgrown before age 3