Domain redirect math?

[TREWS]

I’m thinking about buying the domain of a former competitor who is getting out of the business.   His site has quite a few links, a healthy link profile from several hundred domains (a few common sites, but mostly different) and a Domain Authority of 44 (mine is a 55).  His anchor text portfolio is very appropriate to my business, and the domain contains one of my most important keywords (one that’s in my own domain).  His site ranks for a many of the same keywords as mine.  Where mine is #3, his might be #4.

I don’t want to operate his business (there’s a reason he’s getting out), but I was thinking that redirecting the domain to appropriate pages on mine would give a boost to the many long tail keywords which constitute a considerable part of my business.

The question is how much of a boost?  How much should I be willing to pay for his domain?

If I get x visits/year from organic search and he gets y, and both sites rank for approximately the same terms, how much would you expect the two sites together get?   Is there a rule of thumb for calculating this kind of thing involving some kind of logarithmic math?  Does the combination of links of similar sites make 1+1=2, <2, or >2?  Or is the only way to make this estimate to go through each of the major keywords trying to figure out what will happen to each?

If, for instance, on average we combined our #3 & #4 rankings and got a #2 ranking, that would probably increase my clicks by 50% because of their higher SERP prominence.  Based on my research, a visitor to my website is worth about $14.  Assuming that his natural search visitors would be worth about the same amount if directed to my site, I would probably make 1/2 x * $14/year.

I'm least comfortable with the first assumption.  Is there a better way to evaluate this?