Lets see the numbers...
Lets say the LED is 100w less than the T8.
OK I'll play
I'm going to pick out a couple fixtures I considered for a shop that I wired a few weeks ago. The fixtures are pretty close to 100w difference.
For the fluorescent I'll use a lithonia IBZ 632 WD. ~18,000 lumens, 218W, $112 with lamps.
For the LED I'll use lithonia IBG 18LM ~18,000 lumens, 114W, $242
Lets say the lights are used about 2 hours every day, I am going to assume this is optimistic for a hobby shop since there might be some days the lights don't get turned on at all or only for a few minutes but there will probably be more hours on weekends ect. If some one can prove they average more than 3 hours a day then my hats off to them. Most people I know don't spend alot of time wrenching during the week if they work a full time job. Obviously If you are operating a business and the lights run 12 hours a day then the payback will be much shorter. The numbers are easily adjusted depending on the use. Perhaps we should have a poll and see how many hours / days a week everybody uses their shop.
And we'll say electricity costs 11 cents / kw (that's on the high side for my area).
The T8 will cost $17.51 a year.
The LED costs $9.16 a year.
So the LED will save $8.35 a year per fixture. But the LED cost $130 more. So I guess it's not the 30 years that I threw out earlier but 15.5 years is still quite a while. I've seen quite a few people putting in new lights in their shop after 20 years.
Also the LED I used as an example is pretty efficient at 160 lumens per watt. Some cheaper fixtures are not as efficient.
For the record I went with the led's in the shop referenced in the beginning. That shop won't even run the lights 2 hours a day, it's mainly a toy shed, but money was not a major concern on the project. I was very pleased with these fixtures. The IBH series that my supplier tries to sell me before sucked.