If your budget is that tight, you may just want to skip the infant seat and use a convertible. You can find them for under $90 and they will last years instead of months.
If you are stuck on an infant seat, then I would look into the Cosco Comfy Carry Elite or the Safety 1st On Board 35 . I highly advise not to get a rear-adjust seat. All the cheaper snugrides are rear adjust and the Comfy Carry is unless you Elite. I believe the Evenflo Discovery states that the handle must be back, which makes it incompatible with some cars, because it takes up so much room.
For head support you can use rolled up receiving blankets on the sides, and that will work perfectly fine. I personally found most of the Graco head supports useless. The Snugride 30 the Little Apple posted is a good deal. It is not a rear-adjust so that is good. It may have an older date, which could be why it is so cheap. Albeebaby has good customer service though, so you can always call and ask if you are worried about how old it is.
For convertibles, you could look into the Cosco Scenera, Cosco Apt or Safety First Guide 65. All fall under $100.