It is going to depend on: 1) where you are located; 2) how much competition you have in your area from other shoppers; 3) how good the reputation you built is with the companies you are working with; 4) what kinds of shops are available to you; 5) how hard you are willing to work and 6) how much time you spend looking for jobs.
Video shopping seems to pay a lot more (for now at least) than other types of shops. With video shops you will earn more if you have your own equipment. My understanding is that the average video shop pays around $40, so 2+ video shops per day should get you to your goal.
On the other hand, most mystery shops pay in the $10-15 range and usually out of pocket expenses to be reimbursed. I don't know that in my area, with the current economy, I would be able to achieve $2500 per month in fees even if I was taking shops that cost me more in gas than the fee offered. There is definitely a 'law of diminishing returns' in shops. If the shop is not profitable for me, I don't see any reason to perform it and write the report. For me, 'profitable' includes useful reimbursements that are substitutes for paying for the goods or services myself (i.e. groceries, gas, oil changes, restaurant dinners).