Ultra balls have an x2 multiplier, regular Pokeballs have a x1, not that big a difference. It's not that hard to catch legendaries if you know how the catch formula works and you do the best combination you can. What do you mean you don't buy it? I have, it's easy!
I've never needed more than 20 turns to catch any Pokemon, legendary or not, except for those goddamn Beldums since you can't really lower it's HP without it fainting (only knows Take Down)...
And nice! I've found a good deal of shinies so far (currently have saved in a wormhole with shiny Crustle, but I've found about 9 shinies so far, wormholes are great!