Honestly, it depends on the quality/depth of service you're looking for and how extensive your AdWords program is.

I've written this up in detail on Quora: https://www.quora.com/How-much-does-a-good-Adwords-consultant-cost/answer/Igor-Belogolovsky-1

But I'll give you the highlights:

- You can probably get someone mediocre for a few hundred bucks a month, but it may not be much better than optimizing on your own

- You can hire a variety of skilled agencies or hourly consultants for a few grand a month

- If you want top talent and have a significant budget, expect to pay as much as $5k to $30k per month

Good luck!

Igor