My definition of "nice living" may be different from yours, but I would say you cannot make a "nice living" by writing a commercial module for DNN.  Writing a suite of modules? Maybe.  Writing a DNN module version of a custom application? Maybe.  Creating a DNN solution for a vertical market? More probably.  Using DNN as the basis for a wildly popular portal used by a valuable marketing segment?  If Google hasn't already bought several like you, quite possibly.

DNN is a tool.  The person who uses the tool to create something will have more opportunity to make a "nice living" than the person who makes a part for the tool.

Jeff

Perhaps (and probably) an unfair question... but I'd be curious to know some general numbers for those that are currently selling modules.  I would agree with all the comments that it is quite unlikely to make a good living, but as supplemental income it probably doesn't hurt much. 

What I'm curious (if it isn't taboo to ask) is how many sales you (that is, module developers in general) have from places like SnowCovered and more recently DotNetNuke Marketplace.  Obviously some people can bring in a fair amount of cash when they have a very ingenious new product that functions well and has a huge niche in the market, but on the same token, I'm sure there are several modules out there that are lucky to even make a single sale.

I'm laying the finishing touches on my first commercial module and plan on releasing it soon, but again, really curious to know what I should expect.  Obviously that question warrants a completely speculative answer that may hold no merit, but curiosity overcomes me.