I have the 870 slug gun. Great gun and puts the deer down hard. Due to a last minute foul up with my scope (My fault) I used a buddies Mossberg 500 combo. I boresighted the barrel with a laser boresight (A must have) put 3 shots through it the day before the season and was dead on and smoked a nice buck opening day. I just did a google search on it and saw this. Its pretty cheap and I know D icks has a sale on them every year as well. Adding a decent scop and mounts shouldnt run you more than another $100. Theres actually the same model out there that has a scope included with the mounts. The scope isn't too good, but you can switch to a different scope fairly easily and can have a good combo for less than $400 bucks.

http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46910