If you're planning to tie tzitzit on yourself, there's an easy way to save money on Ptil Tekhelet in some cases. 

A set of white strings typically costs you somewhere in the neighborhood of $10-$12. In comparison, a set of techeiles strings, which includes white and blue strings, retails for something like $60 to $80 for thin and $70 to $100 for thick. Obviously the difference in price is mostly the techelet dye.

Many years ago, the Ptil Tekhelet Association (back then they weren't yet incorporated) realized that a lot of techelet is going to waste. Usually when you tie tzitzit, you're left with the long string (the "shamash") still longer than the rest. Most people trim it to even the length. Sometimes you cut off several inches, depending on the tying custom, and how tight you tie. With all-white strings, it's no big deal, but with techelet strings, that's a lot of money down the drain!

So Ptil Tekhelet decided they would make shorter strings, to save money for those who aren't going to need the full length.

Short Ptil Tekhelet strings

Here's the math: For the most part, the more times you wind the blue string around, the shorter it gets. If you tie according to the Chabad/Arizal custom, you're wrapping the blue string around a total of 37 times, whereas if you tie according to the Gra or Chinuch or Raavad, you're wrapping a total of 18 times. So they make a shorter set that's long enough for the latter tying methods, and costs about 10% less.

Then there are tying methods that use even less blue, namely Rambam 7 and Amram Gaon and a few other customs. Here the savings can be more than 20%.

Converting a Raavad set to Rambam 

For those who are enterprising and really can't afford strings, there's a way to get discount of almost 50%(!). Let's say you follow the Rambam's approach (one of eight strings blue), but buy a set of Raavad strings and an extra set of all white strings. You take the four blue strings in the pack and snip them in half. Then you take each string and tie it to a white string. You've now converted your set of Raavad strings into two sets of Rambam strings. Is it kosher? Yes, you can tie two strings together, as long as you do so before you attach the strings to the garment. Does it look okay? Well, it certainly detracts from the נוי מצוה somewhat, but it's not very visible, because where you made your connecting know is going to be in the tzitzit hole, and probably tucked into your pants.

Reused techelet strings

One final way to save is to reuse your strings. After a few months the white strings will start to look grayish. Obviously the blue strings will also be dirty, but the dirt doesn't really change the color visibly. So you untie the strings, toss out the white, wash the blue strings by hand (this is really easy, since it's a tiny amount of material), hang them to dry, buy a new pack of all-white strings, and retie them. The tips might be frayed, so you'll probably want to trim off a tad. And voila! your strings now look good as new. They only thing is that you have to be fairly experienced at tying, because you have no extra length on the blue shamash string.