The reason they will give you is cristolube vs silicon and viton vs buna. I find the difference to be less than pennies per cylinder, for instance, I just threw away 3 tubes of christolube that had expired before I used them, and they have about a 10 year shelf life. I only use viton and I only use christolube, so the point is moot. I also have a tank inspection light that costs $50 more than the white light, and I clean everything with blue-gold instead of simple green, but either works as well. A gallon of blue-gold lasts 5 or more years.
But is it cheaper than taking the class and buying the tools and materials yourself? And ask yourself: Are you mad about it because you think it's a ripoff, or you think there is no value, or you just don't understand the process. Once you understand the process, make the decision to run through the process yourself or just pay the extra 20 bucks.