Seats range from $40 to $500! Under $100, I would recommend Cosco Scenera, Cosco APT, and the Safety 1st Guide 65. The Guide 65 installs really upright, so it fits well in small cars. I have one now as a back up for my 10 month old, and it gets the job done!
Avoid any seat that markets itself as a 3-in-1. They are outgrown far too early in RFing mode as you want to get as close to age 4 as possible RFing. They also make deadly boosters by positioning the belt high across the soft belly when it should be low on the hips. They are jack of all trades, master of none. They are never recommended, and there are always better options!