How do I use SourceForge.net for a low cost CRM?

Code canyon sell cheap CRM (30$ to 50$) web application that you can host yourself on your server or using share hosting company.

Then share hosting plan can cost you 5 to 30$ per month.

For unlimited user.

Then there is CiviCRM, which is FOSS, and extremely powerful.  It is also extremely customizable, with a healthy community of developers available to help extend it into any specific needs you may have.

The disadvantage to it is that the UI is a bit unpolished, and it has a bit of a learning curve.

That will depend heavily on the learning curve inherent in the software, the competency/literacy rates of your sales staff, and the training you have in place to address both.  This concept applies equally to commercial software and FOSS, though commercial software adds the retail price into the equation.

For learning curve, there's a bit of research for you.  Look at the complexity of the software, vs. what needs of yours it actually fills.  How is the UI presented?  Are there clear, visible work flows?  Does the user have to interpret the available actions they see, or is the next step generally intuitive?  Is the UI consistent, e.g., are the "Cancel" and "OK" buttons always in the same place?  Does it provide high-visibility feedback, especially in the event of an error?

Another part of uptake is how quickly your staff can get up to speed.  Are you hiring people with strong computer literacy?  Are they able to learn by themselves, or do they need hand-holding?  Do they learn by rote, or by applied concept?  Consider this a sort of multiplier to the previous paragraph.  Intelligent people can reduce the learning curve for new software, just as not-so-intelligent folk can increase it.

Finally, your training regimen is a way to mitigate "damage" from the other two categories.  Goal-oriented training can help staff learn explicit, dogmatic steps to accomplish certain tasks.  This can get a new employee functionally competent quickly, and almost eliminate any UI-based issues, but it also limits exposure and passes problem-solving up the ladder to supervisors.  Well-rounded training can decrease the amount of investment necessary to overcome the initial learning curve, and give general familiarity, but takes longer to achieve desired results.  Whichever regimen you implement, it comes with its own cost in time and resources, both initial and ongoing.  

So, the short answer...  free software is free.  Retail software will come with its own price tag.  All other ROI considerations are too specific to your business to effectively answer here.