It's a problem every business faces but you'd be amazed the amount of businesses who have no filtering and sit for hours each day manually deleting spam, which you don't need to do!
It's also in the interest of webhosts to provide spam filtering , so you'll probably find most offer basic filtering for free. Basic filtering should be sufficient for most businesses - while larger enterprises will need to pay for an advanced solution.

Spam filtering always takes a bit of tuning before it'll work perfectly, and the spam you receive changes, so you may have to 're-tune' every so often!

I'd recommend you look for a hosting package with SpamAssassin, BoxTrapper (or similar packages), user-level and account-level filtering. These should be free - I wouldn't fork out extra money unless you've rigorously tried free spam filtering.