UPGRADE SKETCHES EQUALS A POOR INVESTMENT
I've realised just how poor value it is to invest in sketches - at least for resources.
Consider : 10,000 drachmas buy 1,000 sketches. Now, for lumberyards and forges to upgrade from say level 21 to 22, you need 200 sketches per upgrade. Hence 5 buildings upgraded cost 1,000 sketches ie 10,000 drachma. For the same 10,000 drachma, you can buy 7.5 million resources (10,000 drachmas buying 250 lots of 30,000 res at 40 drachma per time) - this 7.5 million has ignored any WOF special offers.
The building upgrade route would give an additional 100 per hour and so 5 buildings = an additional 500 res / hour being generated. This means that it would take 15,000 hours (or 625 days) before you have generated the 7.5 million resources.
My conclusion : as a payback project then paying for sketches to upgrade resource buildings is a terrible investment. Farms are an even worse investment as level 21 to 22 needs 260 sketches per farm; do the maths and it is 813 days payback.
Oh, and lets not even talk about the infirmary - paying massive amounts to revive offensive units which, unless you have screwed up, would never be killed in your city.
I've rambled on a bit - but it is raining here and so nothing else to do :)