L.,
If you can wait until he can face forward and skip the convertible seat, you can save some money. Forward facing booster seats go up to 100 lbs instead of 80 lbs for most convertible seats.
Whether or not you need a convertible seat will depend on how comfortable your son is in the infant seat and how big he is by 12 months.
I got the Evenflo Triumph 5 ($120 new) which was much cheaper than the Britax Roundabout ($220 new). The evenflo is rated second after the Britax roundabout in overall ratings but the evenflo is actually rated higher in crash protection than the Britax. The reason the evenflo is rated 2 points lower than the Britax is that the straps get some getting used to.
If you can go directly to a regular booster seat, the top rated one by consumer reports is the Graco Turbo Booster seat.
models numbers:
Graco TurboBooster 8493 $50
TurboBooster SafeSeat (Step 3) 8496 $50,
TurboBooster SafeSeat (Step 3) 8497 $50,
TurboBooster SafeSeat (Step 3) 8498 $50,
TurboBooster SafeSeat (Step 3) 8673 $60,
TurboBooster SafeSeat (Step 3) 8674 $70.
If you decide to get a convertible seat, get one that goes up to 80 lbs (4 years old aprox.) and at that point you can get the Graco Backless TurboBooster for $20 (highest rating for backless booster seats).
I hope this helps,
A.