When I was taking Finastertide (generic Propecia Strength specifically for MPB, hence cosmetic) at Costco it costs me $28 for a 90 day supply without insurance but with a prescription discount card I got for free at Costco. My HRT doctor switched me the higher strength version (generic Proscar that's 5 times the dosage of Propecia, but is for shrinking the prostate.) It is cheaper at $12 for a 90 day supply at Costco. (I believe Wal-Mart is a few dollars more, but not by much.)
Shop around if you can or ask doctor for the higher strength version because it's a lot cheaper and if your already going to be taking HRT, it's not going to cause any side effects that Spiro is going to already give you.
Now is it necessary, well I think it maximizes your chance to have a full head of hair. It made a huge difference for me.