How many of each tip do you have? Does it vary with each style/particular tip?
So far I have about 20 metal tips, a few of them are specialty, one of each.
I have the cake mate tips for icing tubes, just one set of 4 tips and a flour nail (got it at a discount grocery store that sells out of season things [it was a few days after Halloween when I went I got the tips and 2 tubes of black for $1.25)
and I have a Progressive decorating set with 8 tips. (a nice heavy plastic)

My best suggestion is instead of trying to buy all the different tips out there figure out what you use the most. I have about ten 3s and 16s as well as many 12s, 21s and 104s.

I think it is best to have mutlpies of numbers you use alot that wy you can use several on one cake with out having to clean the different colors between use and if you do a couple of cakes back to back you can save washing tips until the end.

I also like to have some varetiy in the sizes so I do have atleast one or two of most of round tips as well as star tips, and petal tips. That way you can do a little larger or smaller when need be.

I've actually got several of the big cake icer tips as well.

And we won't even discuss how many couplers I have....

If you're planning to make cookies, you will want lots of 0's, 1's, 2's.

this will be different for each person and how they decorate. I use alot of 6's and 4's. Also 21.