The job depends on:
Are you doing it, or a contractor? If you, what is your skill level? If you, does local code allow you to do it yourself? Some jurisdictions forbid the homeowner from doing gas line work. Note: most gas grills are set up for propane - the two gasses aren't simply interchangeable. You would need a conversion kit for a grill, which would contain different orifices that would need to be replaced for each burner, and probably a different regulator. Further, code may dictate whether or not you could branch off from where your fireplace is - though YOU may not plan on using both at the same time, code may require that the line be sufficient to supply both at the same time. This is, I believe, calculated based on the BTUs of your two appliances, along with the size of the line and length of the line.
In my opinion, it's not worth the bother, unless you're redoing a patio. An added benefit is that propane burns hotter than natural gas. I have four or five propane tanks; I fill them in the Spring and rarely run out before winter. Fill ups are around $10 each. (Don't do the exchanges, they're a rip-off.) Heck, I have propane at my house, and have put in all but one of the gas lines myself, and it's not worth the bother (to me) to run an additional line to the patio - I'm sure the time investment to research exactly how it needs to be done, as well as do the project correctly would be about equal to a decade worth of getting my propane tanks filled.
If you do decide to go that route though, you'll definitely need to find out what's code in your area - materials, depth, shut-off valve locations, can you use a quick connect fitting for the grill, where is your line going (under ground? That'll have its own set of requirements), etc. In addition to black pipe, CSST (corrugated stainless steel tubing) may be an option. I presume that for an outdoor application, it has to be run through a protective conduit. But, it reduces the number of fittings, as well as installation time. If you go CSST, it needs to be bonded to your home's ground with solid #6 copper. So, not only is the location of the gas line a consideration, but there could also be electrical considerations. For a long time, it was next to impossible for a homeowner to get CSST - several of the brands still insist on it being installed by a pro who has taken a short course on how to install it - I didn't have a source for it until about 2 or 3 years ago. It's not hard to do, but the fittings for the ends are a bit more complicated than simply screwing together black pipe. Additionally, there's some sort of plastic that can be used for underground sections of line, but never above ground. PCV, I think. (Not PVC) - I have zero experience with it, and your local code would dictate if you could use it.