In my honest opinion, Frosties are NOT worth it.
I am a firm believer that time = money, and I usually go the fast, lazy route to achieving any sort of stat that I want, no matter the cost. But in the cash of doing prayer, Frosties are just so damn expensive that to me, the cost is not worth it.
It would be much smarter to go with Dragon bones, or even infernal ashes. These might take awhile (atleast a good solid week of playing all day), but you will save MILLIONS. Back when I get turmoil, I payed 70M for 70-95 prayer because I bought infernal ashes. The difference with my situation though, was that I had many more reqs to fill before I could even GET turm, so I just plugged away at those slowly while I ran ashes in between.
I would suggest calculating up the costs, and the amount of item you would have to carry to the altar for each item. And then calculate up the total time it would take you to run there (in hours), and figure out from there what is the best choice for you, the time you have to play, and your budget. Good luck!