Taxi is way different. With Taxi, you go through a middleman for your CHANCE to be heard by a supervisor and land a placement. You pay CDbaby or Tunecore, and they put your music on all those sites - it's not a submission process where you have the chance to be rejected.
I don't know how much is involved with applying to sell your music on iTunes by yourself, but if you take into account the fact that these services put your music on dozens of sites... i mean, think about the time and effort it saves. That's worth money to me... and it's not an arm and a leg to do it, either.
If you have faith in your music and yourself, you should be thinking about how much money you're going to make BACK after your initial small investment to market your music properly.
If you'd like to sell your own releases.. just use Bandcamp or ReverbNation or something. They have purchase options for your listeners. However, those choices aren't as "sexy" as iTunes, but you're either gonna be a DIY artist or you're not. As a buyer, I personally don't care whether I get your music from Apple or Bandcamp. Just as long as it's good music.