2500 certainly isn't a TON of MQMs, so you should be able to do this for around $350 without a lot of effort.

One option is that you may be able to purchase MQMs from Delta at the end of the year. Last year, Delta was selling 2500 MQMs for $395, which isn't too far out of your price range. However, IF Delta does offer MQMs for purchase this year (and they may not), the price will almost certainly be higher than it was last year. Still, if you want the miles and can't find a flight that works for you, this may be an option.

That said, I am seeing lots of flights from BTV to SEA for around $320 at the end of October. You can fly BTV-DTW-SEA-DTW-BTV, 4928 MQMs for 6.5 cpm on a same day turn. It's not the BEST MR deal you will find, but it will serve your purpose. If you want to add some stops, you can push it down to 4.8 cpm (BTV-ATL-LAX-SEA-DTW-LGA-BTV for $328).

Two tools that can be really helpful when planning mileage runs:
Matrix flight search (especially in calendar search mode)
Google flights (especially in map view)

Good luck and welcome to the MR forum!