The least expensive way is to use fresh lemon juice. A lemon costs less than a dollar at the grocery. (Don't use the bottled stuff, it has extras you don't need like water and preservatives.)
http://www.ehow.com/how_2179867_lighten-hair-using-lemon-juice.html
Another way is to get a bottle of hydrogen peroxide from the drugstore. It costs less than $3. Mix it in a bowl with some conditioner to make it thick if you like. Paint it on, wait 1 hour (more or less depending on the color you want), rinse with cool water. Condition and rinse again.
Great advice for applying any type of color yourself can be found here...
http://killerstrands.blogspot.com/search/label/Highlift%20Blonde
If you decide to DIY with a Sally's box kit, do a protein deep treatment a few days before the highlights.
Also you will need a pH correcting treatment for after the highlights.
A pH balanced moisturizing shampoo that is color safe. No sulfates.
A moisturizing hair mask that is color safe.
And a deep conditioning treatment that you use once every week religiously.
Best wishes.