I don't generally do covers, but I have a few including this one from Martinique to Barbados with 8 Francs worth of stamps on it (3 of SC#163, 2 of SC# 161 and 1 each of SC#139 and SC#141).
Ignoring the folds, stains, and general malaise - My 2015 Scott Classic Specialized Catalogue suggests the "most common procedure may be summarized as follows: the stamp which has the highest "value on cover" is counted with its on-cover value, while the other stamps are added on to the total with their normal value as stamps "off cover".
The 161s and the 163s are listed at $45.00 on cover and 80˘ for used singles while the 139 is $15.00 on-cover and the 141 is $8.00 on-cover, with both at 35˘ for a used single. So I come up with a catalogue value of $48.90 [$45.00 + (5 x 80˘) + (2 x 35˘) = $48.90], if only it was in perfect condition.
Does that jibe with the consensus here?
With all that being said, I'm going with a price of about $4.81 on a good day!