If you can't afford the 2500 a month, you should probably take SEO off your priority list. Done right requires building and nurturing relationships long term with bloggers, webmasters, journalists, associations, etc... so you have effective distribution whenever you create content that can't be found elsewhere... That investment is more like $5k/month for 18 months to see breakthrough results...

Depending on your CMS, your site is probably 90% technically optimized, and there are a million cheat sheets for onsite best practices, but if you really can't afford it...

Focus all your money and energy on improving your user experience. This will not only squeeze more value out of your existing traffic, but will also help your SEO long term.