One-way backlink on competitors sites

Hey guys,

I'm just weighing up all the positives and negatives with this but want to know how much this would actually help my SERPS.

EG: example my micro niche domain whiteweddingshoes.org "white wedding shoes" ranking #1

I own a domain that is ranking #1 for my main keyword eg(white wedding shoes) which is a micro niche and exact searches from Adwords are around 40k. But am ranking for the more broad keyword "wedding shoes" in that niche on page 4 which gets around 200k exact searches.

Now when searching for wedding shoes I ended up contacting all the sites that I think would sell advertising. I ended up with 1, weddingshoes.org which is ranking on page 2 for that keyword and is wanting me to pay $200PA.

I am hoping by buying the spot, will help my SERPS and push it from page 4.

Now my question is: If weddingshoes.org has one-way anchor links within his article to my site, how much juice would this give me? I mean it will be hard to tell but does anyone have any past experiences with this type of link building?

Thanks.