I just posted this for someone else looking for a convertible in a small car, so just pasting the same reply. Hope you find something that works! The less expensive seats out of the list would be the Triumph, MyRide, and Guide. You might find a Diono Radian R100 for a little over $200 on sale. Angle Adjuster is less than $15 and sold separately.
For convertibles, this blog post might be useful, except many seats aren't available in Canada (I'm assuming you're in Canada):
http://carseatblog.com/22818/the-ul...ertible-space-comparison-review-size-matters/
The Britax Click Tights would be one to try, although it has different acceptable recline levels based on the child's age/weight so keep that mind. The Boulevard and Advocate have higher harness height than the Marathon. Others to try would be the Dionos with Angle Adjuster (Angle Adjuster can only be used once child has complete head control and can sit up unassisted), Peg Convertible, EvenFlo Triumph, Graco MyRide, and Guide 65, again making sure you have it set for the right estimated age/weight when you are trying it.