How much does it cost to replace a DSLR shutter?

It might fail on your D5000 soon, or it might not. When it does it might be expensive for what you need, or might be a case of it's time for a new body. I'm not saying a point of failure is an excuse for an upgrade, but the repair price of a shutter in a D5000 is probably getting close to the price of a second hand model at that point (it's probably a long way off after all!), so if and when it does fail it might be worth looking at whether a replacing that shutter is cost effective compared to a new body.

For a Canon XXD series, generally a new shutterbox repair runs $330

While I have heard of a few D5000 users encountering Error. Press shutter release button again., indicating a shutter failure, this is rare. Considering your usage, the shutter should last at least two to three years. If and when the shutter fails, it would probably cost about $200 to $300 to replace, but that's a large part of the value of the camera body and you may want to get a new camera at that point.

Note that Nikon says "tested to over 100,000 cycles" (emphasis added), so the shutter should be expected to last some time past the 100K mark and probably should not be expected to fail at just over 100K, but likely somewhere between 120K and 150K. Furthermore, this is a mean time between failures; your shutter may last longer or shorter than this number of cycles. In fact, there have been brand new D5100s with this problem.

This is the biggest problem of the fast shutter speed cameras Canon says the shutter lasts up till 100K, but the good news is you might be lucky enough to use your shutter till 200K or even 400K, but in any case, you can replace it for roughly 300 USD if it fails early

However, note that you should never replace your shutter if the repair price exceeds 30% the price of the whole camera, so in order to get a better solution, sell it when it reaches 70K — and dont forget to honestly inform the buyer about the shutter count — and get yourself newer model so you will always be updated by the latest models without involving yourself in the hassle of repairs. Hopefully, you will be changing the camera body without the lenses and don't need to change the lenses as long as you protect them well by UV filters and seal them well in your antishock camera bag.