Well, if you're going to be replicating something like this:
Your parts list looks OK. You could probably get it cheaper if you look around (SparkFun isn't exactly the least inexpensive option); especially if you are in the US where we have nearly a plethora of surplus electronics dealers. Certain items like the buttons, the potentiometer, and the midi connector are easy and cheaper to come by; but do your homework.
I know that SparkFun's resistor pack isn't that bad of a deal, as far as new resistors go (until you start buying really large quantities); if you were to do surplus, though, from All Electronics:
You could get twice as many resistors for less than what you would pay for the same number (ie, 2 of those kits) at SparkFun (ordering that many isn't as easy with All Electronics interface, though). But the difference isn't much (about $4.00), and SparkFun's resistors are "new", instead of surplus - so all in all, its a fair deal. The numbers flip, though, when you start order 100 pcs of each value (I did that recently for my own bench because I didn't want to run out of values any time soon - did an E-24 decade series; it wasn't cheap, even doing it surplus, but I won't run out any time soon!).
The other parts you should shop around for (you might be able to get the breadboard and jumpers cheaper on Ebay if you are willing to wait some). Oh - if you don't already have one, get a multimeter; it is an -essential- tool. If I had to choose between a multimeter and a soldering iron, I would go for the meter. Since you aren't doing any soldering immediately, you may want to look into getting a different tool kit; buy a separate meter, and small diagonal cutters and pliers - thats all you'll need for now. The soldering iron can come later (and you really want something a little better than what is in that kit - you don't have to go all out and buy a super-expensive iron, but there are plenty of cheaper soldering stations out there that are much better to have).
Lastly - do all of your development with your regular Arduino (you do have one, I take it?) - that way if something goes wrong you only likely "blow up" a cheap DIP ATMega, instead of a difficult to replace SMD part - you might even want to forgo that device until you get a breadboarded prototype completely working and debugged; then buy the smaller device plus your other parts (maybe the soldering iron, and prototyping solderable PCB - you didn't mention what you were going to be using a soldering iron for, or how the soldered MIDI device was going to be put together).
Hope this helps...good luck!