Une suggestion
Expression habituelle en astronomie / physique, mais aussi dans: www.pme-prf.gc.ca/eic/site/sme_fdi-prf.../01172.html

mid here means middle, as in mid-June. Present value is the value on a given date of a future payment or series of future payments, discounted to reflect the time value of money and other factors such as investment risk.

So, for each product, say, 1 year staring from January and ending in December, the mid present value would be the value for each product at the point of around the end of June or beginning of July.

http://en.wikipedia.org/wiki/Present_value
http://www.thefreedictionary.com/mid

I believe this is the short version of 'Mid-Period Present Value Factor. Present value factors used are for mid-period (i.e. the mid-point of a cycle) to average the flow of money over a year.
E.g.
"The annual net cash flow discounted at 10% using mid-year discounting is $2,859,298 Table A Case 4). This value is two-tenths of 1% different than that obtained using the monthly calculation with effective monthly interest rate. The annual net cash flow discounted at 10% using end-year discounting is $2,679,697 (Table A Case 5). The 6% difference in net present value is due solely to the discount point assumption."

There is a fairly detailed article (prepared for attorneys and judges) at the first link below, which might help you decide if this answers your questions; "The Income Valuation Approach."

The second link takes you to a much more complex circular from the White House, "Guidelines and Discount Rates for Benefit-Cost Analysis of Federal Programs." Sample paragraph:
"The present values of costs and benefits computed from the table above can be converted to a mid-year discounting basis by multiplying them by 1.0344 (the square root of 1.07). Thus, if the above example were converted to a mid-year basis, the present value of costs would be $110.06, the present value of benefits would be $147.31, and the net present value would be $37.25."

I hope this helps.