Realtors always think in terms of resale and investment.
You really want to "invest" the appropriate amount of money so that the house will bring you the best possible return on your investment. You can always realize incremental savings on individual items, but your overall budget should be based on the calibre & price of the property "above grade"
I think it's important NOT to overbuild the basement (compared to the main floors "above grade")
For an extreme example, you shouldn't put a $75k basement into a $100k house!
At the same time, you don't want to skimp on important features -- a nice bathroom is at the top of that list.
I like to use a 10% rule: If you have a $250k house, $25k in the basement is appropriate. On a $750k house, a $75k basement is appropriate. Every dollar above that will have diminishing returns. So, just like swimming pools, you can add more but you probably won't get it back when you sell.
I also agree with the $30/foot "ballpark", but that varies widely from location to location. Plus, contractors are VERY hungry in certain markets right now and are often willing to work for much less.