Britax seats are pricey, but children tend to use them longer than other seats. The Roundabout, Marathon, and Wizard all go to 33 pounds rear facing, the RA goes to the regular 40 pounds forward facing but has higher harness slots to accommodate taller children longer, the MA and Wizard go to 65 pounds (yes, in a 5 point harness) and have the highest harness slots of ANY other convertible carseat. This means that while you do spend more money initially, it eliminates the need to buy a combination (forward facing harnessed seat that converts to a belt positioning booster) seat after your child outgrows a regular convertible carseat. Most convertible carseats have a top harness height around 15-16 inches, and most children are not ready for a belt positioning booster when they outgrow their convertible by height. The Marathon and Wizard have top harness heights of 18 inches. Most children can use those convertibles until age 5, easily, and many make it even longer. Then you could buy a relatively inexpensive booster seat.
If you were to buy a Cosco Alpha Omega, for example...that would cost you around $140, give or take. You would then need to buy a combo seat when your child grew too tall for the harness in the AO (or switch to belt positioning booster, since it IS a combo seat, but has extremely low top harness slots, my 2.5 year old dd has already outgrown the harness mode at 38" tall). A decent combo seat is the Graco Ultra Cargo, it has harness slots 2.5 inches taller than the Alpha Omega, at 17 inches. It is the combo seat with the tallest harness slots. It will run about $70-$80.
On the other hand, you can get a Britax Marathon for around $210 online with free shipping, or for $250 at Babies R Us. It has many more user-friendly features than any other seat on the market, making it WELL worth the price. Not only that, it has harness slots 2 inches above other convertibles, and accommodates 25 more pounds harnessed than other convertibles.