Please keep in mind that you're proposing advertising an incomplete product. This isn't a car or a house, it's software. Regardless of quality, feature, or price (or lack thereof), you should ask yourself what the purpose of such an endeavour would be as well as the moral and legal issues regarding it.
In addition, what would the average person think about when he hears a spot about a relatively (hugely) unknown independant game in some backwater radio station (2-10 bucks for 30 seconds?)? Would he actually give a damn? Are you sure it wouldn't actually hurt credibility as it resembles "selling out"?
Furthermore, what's the payoff matrix for such a project? Would the projected increase in traffic and prospective donations be significant enough to exceed if not cover base advertising costs? I find your 2-10 bucks for 30 seconds far too conservative and for reasons I will illustrate below, unrealistic.
Advertising is pointless if its without aim or reason. What is Dwarf Fortress? It's an ASCII roguelike game that is excruciatingly (and fantastically) complex. It's main audience consists solely of overeducated, bored nerds and geeks. It is also targeted specifically to such an audience (at least for now). What are the odds of such a target market listening to radio at the times where 2-10 bucks for 30 second ads play?
Conclusion: It's far too early to market this game. Word-of-mouth is fine, free is fine, but doing anything extra is counting chickens.
Here's a weblink, which seems reasonable enough, concerning radio advertising costs, suggested frequency, and marketing. It's cheap, but not that cheap. http://www.burkhardworks.com/Radio_Advertising_Costs.htm