Seriously, I have found that overall you get what you pay for. Myabe look at running a PPC campaign? Obviously getting links in place with similar sites will help and maybe you could feature in their newsletter, if you return the favour?
SEO
Forum Posting
blogging
article writing
Email Marketing (need to build an opt in email database first of course)
Cheap =
CPC
Affiliate Marketing
Juat about all other internet marketing (when you compare it to other mediums)
The free or really cheap ways of marketing are generally long term solutions, if you are looking to generate visitors quickly then it's cpc all the way baby!