Sales and Marketing somewhat compliment each other.
Good marketing will get people to know about your product and positions you to sell. You however may not sell (or increase sales) without a properly implemented sales strategy.
A good marketing strategy without a good sales strategy is like a dog that barks but can’t bite.
A few things you can do to manage cost;
1. Always budget before you spend.
2. Carry out monthly reviews of performance on sales and marketing. What are your objectives? How well did you perform?
3. Keep it small until you find the perfect sales and Marketing mix. Don’t overshoot on your cost just yet.
4. You must have a strategy with well defined objectives. Be deliberate with this.