Best marketing ROI when your marketing investment is almost nothing?

Hi All,
I launched an online pet supply business this year. Things have been going understandably slow and I'm okay with that for the most part. But looking forward to next year I want to plan for a budget that includes some money for marketing. Is it reasonable (or at least not crazy) to plan for marketing to be my largest expense for the year?

This year my expenses were primarily startup (website platform, business registration, inventory) and a little bit for promotional materials. Now that all of that is established, I want to focus on marketing next year to get my sales volume up. Here are the things I've been been doing this year:

• posting on facebook, twitter and pinterest (lots of site visits, a few sales)
• working personal networks (lots of site visits, a few sales)
• sending out discount codes (largely ignored/unredeemed)
• vending in person at local pet events (good onsite sales, no followup sales from brochures/coupons handed out)
• one promotion with a major pet blog (cost the most, tons of site visits, no sales resulted)

So I'm getting a feel for what works and what doesn't in terms of low cost marketing. If I budget some money to spend on more formal marketing (like the pet blog promotion), how can I judge in advance if that will be money well spent? I'm guessing I will be able to spend a total of $200 next year on marketing. Can I increase my sales on a marketing budget that low?