Well...I personally have two car seats that are both highly recommended by the car seat techs...and they are very different in price.
The highest recommended seat is the Britax Marathon/Boulevard/Decathlon/Roundabout...they run from about $200 on up, and all but the Roundabout FF and harness to 65 lbs. We have the Marathon in my car.
Our second seat (for DH's car) is a Cosco Scenera...it is a no frills, basic convertable car seat...and you can find them on sale at
Walmart or Target sometimes as low as $39, but usually no more than $55. It only goes to about 45 lbs FF, but is a great seat as far as safety is concerned.
So...it just depends on what you want to spend. But you can get a very safe convertable seat for not a lot of money.