To increase sale, usually we need to increase marketing too. How do we know how much etra marketing investment is enough?

This is the traditional question that the Financial and the Marketing director (and sometimes the sales director) fight for at this time of the year when budget decisions are finalized. 1 see it as a budget expense, the other one as a budget to spend, at the end of the year (or quarterly they check out sales figures). No one can say if 1,000$ invested in social network or CRM software made a profit of 2,000$ or a loss of 500$?
The answer is quite simple but there are 2 problems to solve:
Problem #1- the calculation of Return On Marketing is not easy, it takes the Marketing manager to have a "financial controller mind" and always get the sales figures and feedback on leads at the same time they spend on each Marketing tool. You follow me?
Problem #2: Spend on tool or spend on skills?

Let's start with Problem #1: how to know if your investment of 1,000$ in a Tuoi Tre advertising or couponing campaign made a profit or not. If yes, you should continue. If not you should stop. So you need to start to track each sale and find out which Marketing tool brought you that sale. Then you put the expenses, including staff time and other resources -stationary, phone, telecom...- and you have a Profit & Loss that shows you for 1000$ invested you made 500$ profit after all costs are paid for: Return on Investment 50%. Now what about 1,000$ spent in Social Networking with "Marketer's posting" or a CRM database sending emails to a long list of prospects. You need to establish 1, 2, 3 or more other P&L to check which Marketing tool to push, to add or to eliminate from your Marketing Mix... May be the Social Network will return you 1,000$ profit (100% ROI) on more clients and the CRM will make you 3,000$ profit (300% ROI) for the same $1,000 invested in marketing. This is Smart and hard work.

Then of course comes Problem #2: is the 50% or 100% or 300% ROI due to the tool or due to the skilled employee in charge of the campaign/tool. 1 advertiser making the ad in Tuoi Tre can do a lousy work and get 50% when a top creative/business minded will make an ad that will attract 400% ROI, the same goes for the CRM manager a bad one may pulling zero new clients, a negative ROI!
NB. In doubt, invest in talented people, they are the creator of value/money.