Although especially SQL server seems very expresive MS also has licenses where you pay per month.

Last time I checked that was around 75€ a month, not use what the $ price is.

And if you are really succesful, that shouldn't be a problem ;-)

It's a valid concern.  And one you need to investigate, but you need to do it in temrs of what works for you and your business.  If you're hosting, compare the cost of hosting as you expand.  Compare development costs.  Compare the cost of additiobal labor or replacement labor as you expand, using the skill sets required for each solution.  Compare support costs for those functions you cannot handle in house.

Do all of this now, before you get too deep into the site design and development.  You will essentially be locked into what you choose unless you are willing to step backward and redo the entire project.  You may want to do some simple proof of concept designs to see what's involved.  Make sure you count your labor and time to market as costs, since they are.

DNN is not the solution for everyone and everything.  But find out now, not later.

Jeff