You'll have to look at the documentation that accompanies the sim you intend to use. 61.129 allows you to
Credit a maximum of 50 hours, but the FAA restricts certain FTDs based on their realism. For example, my school has a PI-135 PCATD. For the instrument rating, FAR 61.65 allows
A maximum of 20 hours may be performed in that flight simulator or flight training device if the training was not accomplished in accordance with part 142 of this chapter. However, the FAA restricts a PCATD to 10 hr of credit for the IR.
At our club, use of the sim is free -- all you have to do is pay for the CFI's time. 10 hrs of instrument instruction is only $300, so it's a pretty good deal. There are also things I can do in a sim that I can't do in a plane: move you from one spot to another at the click of a mouse, light the engine on fire, drop the weather to below mins, etc. Most of all, you're airborne in the first 5 minutes of being billed with no before takeoff/post-landing checks required (although you may do them for training). A sim is a valuable tool and you can get good training from it, but it also has limitations. I think the best way to approach simulator training is integrated with aircraft training. Use each training device for its strong points.
Some schools will often throw in free use of a sim as part of a package deal, since the costs of something like a PCATD are really quite low. Ask your local FBO if they will give you free sim time if you get your commercial training with them. You never know...
Now, if you can split time with someone for about $25 an hr, then it's the same price as flying our sim. Flight time is a more valuable logbook entry than simulator time, but you'll probably get much better training from an hour in the sim than an hour as a safety pilot, all things being equal.