I think I can recommend this, Im not using it myself, but this is really the ONLY LEGAL method of gaining alot of followers fast. Most promoters use spam, but as an experienced Twitter user myself I can really say the best way is just to connect with other people. As this takes alot of time to do yourself, I think the 10$ is worth it.
Just my opinion.
Thanks. The method that I use (and have always used) is probably the most legit. Targeted following can really show nice results.