Back-up car seat

I know there have been discussions about car seats so I apologize! We have just bought 2 Graco 4ever all-in-one seats for my car and for DH's truck. After spending a total of $600 we now need a seat for grandmas car. She takes LO frequently so I don't want to have to switch seats all the time but only travels short trips about 10 miles at a time. So I am looking for a less expensive seat that is easy to install! Any ideas?? Thanks