This is Dr Wu. I have been asked to assit you with your question. The starting dose of Effexor is 75mg. However, since you are on a very high dose of prozac, you will likely need at least 140-180mg/day of Effexor ( 20mg of prozac= 74mg of effexor). Since both medications belong to the same category of antidepressants( SSRI), it is safe to switch from prozac to effexor without first tapering off the prozac. However, the switching should be carefully monitored by your doctor, as withdrawal symptoms can still occur. One month supply of Effexor will cost you about 90-100dollars.
I hope this is helpful to you.