New servos can be gotten for less than $3 (**broken link removed**). Standard Hitec servos from a LHS are about $8.00. If you know any modelers, older ones will be free.
A two position drive can be done with a microcontroller (e.g., 12F509) or NE555, which are each less than $1.00 (DigiKey prices). All you need in addition is a switch and a few resistors and capacitors.
The cheapest solenoid on DigiKey is $21.00, if that will work. Others range up to $35. Then you still need the springs, switch, and whatever else will be need for an as yet unfinished mechanical design.
John