Airtronics,737 and 738 runs about $80, the 357,358,and 360 run around $100.
Futaba, the 9402, 9405, and 9451 are great servos and run about $80.
JR also makes a good servo, the Z4750 runs about $85.
They best buy out of this list would be the Futaba 9451, has the speed and torque equal to the airtronics 357. Only thing is it is digital and many people avoid running digital servo's in their nitro vehicles, I use them for my throttle/brake servo and have not had any problems. The next choice would prolly be the Futaba 9402, similar performance to the 9451, but slightly slower and slightly less torque but very capable of handling a nitro trucks steering.