I have a 1997 Ford Contour 2.0liter 4 cyl. How much should I expect to
pay for parts/labor to change a timing belt, serpentine belt,
tensioner, and piston gasket?

Timing belt alone would be about $300 to $400.  Serpentine belt SHOULD
not incure any extra labor, so, $20 - 30 there.  Tensioner, that is a
bit tougher, could range anywhere from 50 to 200.  Piston Gasket? or
Head Gasket?  Can't say as I have ever heard of a piston gasket.  BUT
I would estimate between 450 - 600 MAX for the entire job.  BUT this
will depend on what type of shop or if you take it to a dealership. 
ALWAYS use a mechanic you can trust or comes highly recommended. 
Avoid shadetree mechanics for this type of job!  Timing belt for this
job will require engine removal and if anything is done wrong, can and
will ruin the motor.