Before you go replacing expensive JDM Merc parts (couldn't resist, sorry..), take some time and see what's happening when you try to start it. Look at the voltage drop between the battery and engine block when you turn the key (you'll need a friend sitting in the car), and you might find something very eye opening.

I've had three different cars with the exact same behavior as you're having, with three different fixes that were all cheap or free. One was a cracked battery clamp with only 10k miles on the car), one was ground corrosion where the ground cable attached to the block, and one was corrosion on the connection to the fuse block. The most expensive fix was a new ground cable. All three problems were found with a multimeter and minimal thinking. In the case of my WRX, I had 5V between the engine block and the battery negative post, and the car only had 20k miles on it. Stupid ground cable is attached under the intercooler, so rainwater corroded the connection, but that's not the point.

My point is that you should always verify that the easy to check things are working properly before worrying about spending a single dime. It may be a starter failure, or something like that, but you could just as easily bolt in a new starter and the problem could still be there. Always know what your baseline is before digging out the credit card.