I just took on this project, it took about 2 half days to complete and can easily be a DIY project. I went a step further and did the following:
1) Cleaned entire shower well using a mildew cleaner like mentioned above. I used a dremel with a small cleaning head to clean the small, bad areas even better after cleaning the entire shower with a regular scrub brush attachment you put in a regular drill.
2) Used a dremel to remove about 10 linear foot of grout in areas that had cracked out and replaced the grout.
3) Cut away the old sanded and clear silicone caulking and re-caulked all seams/corners with the correct caulk(either colored or clear)
4) Let the grout cure 3 days then applied a coat of tile and grout sealer to the entire inside of the shower and waited another 3 days before use.
Shower looks great now. Steps 1 and 2 took about a half day. Steps 3 and 4 each took about an hour, you just had to wait between steps so it took a week to complete.
I bought cleaner ($10), caulk ($15), and a pint of sealer ($15). I already had the dremel, razor blades, grout, and cleaning heads. Overall, spent under $50, much cheaper than hiring someone with all the steps and waiting in between to do it properly.
Edit - use that after shower mold preventative cleaner. Spray it on the walls, and especially your area that mold real quick after you shower, takes about 5 seconds and make a world of difference
