chiming in with the opinion that in today's market, a bathtub in the master is no longer a must.
If you were to sell the house to a family with small children, they may want a tub somewhere in the house to bathe the kids.
A tub to shower conversion is not terribly difficult. We had ours...
jessica:
you may be able to call upon the services of a specialty remodeler. Depending upon the area, a dealer for Re-Bath or similar one day bathroom remodelers may be able to help you out without tieing up your house for weeks to come.
I would budget anywhere between $5K and $10K.