Fraza
Silverbills, spice finches, and common waxbills are about the only passive birds that would fit your budget. However, they should have heat during winter, which could cost quite a bit. Spice finches look similar to society finches, so the risk of interbreeding is higher.
Java finches aren't exactly passive, but a pair may or may not be compatible with other birds in an aviary. They may also require heat.
If you're willing to spend a bit more money, consider a pair of European goldfinches. They are usually cold-tolerant and should be fairly compatible in an aviary. If you decide to get goldfinches, only purchase ones that are close-banded; unbanded or split-banded birds could be wild, and therefore illegal.
European Goldfinch