This problem is more difficult than the basic math that has been shared. Let me add this disclaimer, just so I am clear and not misleading you. We are discussing methodology of solving appraisal problems and using your example, we are discussing ways determine adjustments. I'm not acting as an appraiser in this case, but just discussing appraiser techniques. I have not seen the subject property and can't make specific judgments.
Obviously, the SF difference in these two properties is significant. The difference is so large that I would try not to use it as a comp. If there not any other comparable sales available, and I would extend the search area out (especially if this a somewhat rural area) several miles.
It would make sense to use something like this if... other similar sized comparable sales are found, but all of them are on much smaller lots. This sale could be used to help support a large site adjustment for the comps that are similar to the subject in square footage and location, but on small suburban lots.
To get a good idea of the lot value, I'd try to find about 3 2-3 acre lots and find a place in the middle that makes sense. Maybe one sold for $200K, but the other 2 sold for $150. I would put the value nearer $150. On the other hand, if you found 2 at $200K, and 1 at $180K, I would value the lot close to $200K. I would be careful about using 1 comp to determining any value. You should be able to find plenty of support if that is really the market value.
Back to your question.
1.No, that's not how I intended to explain it. Again, the difference in these two is too large and I would expand my search area to find properties that are more similar to the subject. The math is easy enough.
Subject 1500 sf
Comp 1700 sf sold for $250,000 or $147/sf
Half of $147 is $73.5, which I would round down to $70/sf.
The difference in the two is 200sf x $70/sf equals a negative $14,000 adjustment to the comps sale price.
$250,000 - $14,000 = $236,000
The adjusted sale price is $236,000. Then start working out any other adjustments. If the subject is superior, then the adjustment is added. If the subject is inferior, subtract the adjustment.
The subject has a workshop which you value at $5K, and comp 1 does not. The subject is superior. Add $5K to comp 1's adjusted sale price.
$236,000 + $5,000 = $241,000
Comp 1 is on 2.5 acres vs 2 for the subject. The adjustment is determined to be $25,000. The subject is inferior so subtract the adjustment from the updated adjusted sale price.
$241,000 - $25,000 = $216,000
Follow that procedure for each significant factor and multiple comparable sales to reach a value.
Again, I would use 3-6 comparables and would try to find comps that are closer in size.
In a case where you are dealing with acreage property it is ok to extend the search area beyond what is normally used for suburban or urban appraisals. Sometimes, you have to go several miles to find comps. At times we catch some flack for it, but it is better to have similar comps from farther away than making very large adjustments for bad comps that are close.
Hopefully this helps.