Well, I can tell you from personal experience that there are some that charge 200+$ a month, and some that charge 50$. It all depends on the quality of the links that you want. The 50$ package might be cheaper, but it is going to have questionable spammy links at best. If I were you, I would save up your money until you are able to pay at least 100$ a month towards a service like this. Just take your time and make sure that you hire the right person for the job.
I would probably just buy "original and quality" articles, and probably embed my links myself, and post them to EzineArticles, Squidoo, Hubpages, similar forums in my category, free blogs and website services like Weebly, Wordpress.com, webs.com, social networking sites like Facebook, and use some of the articles to submit as guest posts on blogs and websites in my category.
I heard if you get one good backlink from a reputable website like TechCrunch, Gawker, EnGadget, Huffington Post, and websites in your category it is better then a lot of backlinks from spammy sites, unrelated sites, and low quality content sites.
It is going to be a lot harder, but the reward that you get on the time that you invested will be well worth it. It will have a much better impact on the search engines than 100 worthless links will.
It is very hard to get listed on Techcrunch, Gawker and Engadget, but not impossible since these websites tend to find articles on other websites via search engines, Google News, and user article submissions. If the article is great, you might have a chance.
You can try getting a link back from smaller reputable blogs, and websites in your niche/category like Ohgizmo, Techeblog, Computerhope, etc will also help your rankings.
Asking friends you know on Tumblr, Blogspot, Wordpress.com who also run similar topic blogs as your forum to link or mention your link might help your rankings as well.