From a North American agency you won't expect a serious effect on your BOF results of your business.

As mentioned in other answers $2500 is a salary of an entry-level marketing specialist.

When it comes to offshore agencies, you can expect bunch of things getting done with this monthly budget. Since the salaries in Eastern European and Asian countries are much lower in $s

The most important thing you want to get as a value out of this budget is your S.M.A.R.T goal. How many leads does your business need? How many customers you need in next quarter? Next year?

How much % you need to increase your conversion rates? Do you need to cut your bounce rates of your website?

If you have an answer to what do you want to achieve, then request a custom plan from the marketing agency how $2500 is going to contribute to your goal.

How they are going to measure it? Does quality matter for you as much as the quantity?

Get also some specific case studies on how $2500 budget retainer helped companies like yours.

Disclaimer: I am an owner of an agency with 26+ employees. Here is what your get with us with $2000 monthly budget and for $3500 and $5000 respectively if you choose to work with us
http://www.incredo.co/services

Call me if you have more specific advice and consultancy

-Artash