I've been trying to find it for a while, I have three very close competitors and we often adjust prices based on what promotions someone may run or just how their pricing changes. It was easier 3 years ago when I had <100 items in stock but it's near impossible to do manually now.
How much I spent: $14/mo for crawling tool and $500 one time payment to help me set up everything (I contacted the author of that blog for help with one project and he showed how and what to do on skype and I've set up the rest myself) if you're more tech-savvy than I am that's a $500 you won't need to spend.
How much it usually costs:
The best option I've seen was $200/mo for 5000 products. I currently monitor prices on 14000 items across all competitors.
A brief explanation of the method:
You use the crawling tool to fetch prices and product names (or anything else) from any website
You save these prices and product names into a spreadsheet
Repeat the same process with same settings whenever you feel like and see which prices have changed