I assume there are similar information providers for IT services. You'd be more likely to know them than I would. Maybe searching for newsletters and industry associations might lead you to where you want to go. I believe Forrester Research has an IT industry section; it might have your number.
You can get some standard industry ratios in several places. We bundle them with Business Plan Pro. We also get them from Oxxford Info. We used to get them from Integra Information Systems. The Risk Management Association has some of this information, too.
Our legal situation doesn't encourage talking to competitors, but if you can find some similar businesses that don't compete directly, that might help.
By the way, you have your terminology wrong, which will complicate your research. Sales and marketing expenses are different from cost of sales, and referring to them that way, particularly with the COS acronym, will send people off in the wrong direction.
It's not your fault: The usage by accountants and financial analysts is confusing. But cost of sales is direct cost, and sales and marketing expenses are part of running expenses.
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software. He founded Bplans.com and wrote The Plan-As-You-Go Business Plan, published by Entrepreneur Press. Berry is also a co-founder of HavePresence.com, a leader in a local angel-investment group and a judge of international business-plan competitions.