We have a similar situation - I have the Marathon in my car and he got a Graco - something that I am forgetting now - it was $65 bucks - there are a bunch at Target that are perfectly safe (they are required by the federal government to meet certain standards just like cars are) it just won't have extra padding and the bells and whistles.
The Cosco Scenera is a great back up seat for Ruth - and it's on sale for $39 at Target this week. It rf to 35lbs and has a relatively tall shell so most (like 98%) children will be able to use it to 2 years in the rf position. It's usually outgrown by around 3 or 4 years old for an average child. Marcus would have outgrown it around 3 and Isabelle would still fit in it at 4 1/4 years old.
Another option if you want a slightly nicer seat is the Safety First Avenue. It runs about $80 at Sears or Kmart. It also rf to 35lbs and ff to 40lbs. It's has a slightly higher shell and it lasts a little