Here is my comparison:
I assumed 1gold=1beaker=1hammer, and 1 food=1.5 hammer. Why, because on average we have ~1.25 muliplier on each of gold, beaker and hammer. I have already stated why food is stronger than these. Naturally, beakers are a bit weaker since we have better multipliers, so this assumption helps Merc in the comparison. Food's worth is always questionable, but let's try it like this for now.
Communism, assuming 80 workshops on average:
+ ~100 gpt immediate gain (dist maintenance and cheaper civic)
+ 80 fpt (I'll use 1.5 multiplier here, it's questionable)
+ ~50 hpt from 10% bonus
- 4095 beakers
- 15 bpt (on average, since we can't switch to Rep for a while) for let's say 25 turns (Great library obsoletes earlier)
- Parthenon: we lose 2/3 of a Gperson's GPP
- ToA, GLib GPP loss: another 1/3 of a Great Person
+ we get a Gspy, so let's say it's even on GPP front.
Sum: 100+80*1.5+50 = 270 unit per turn
But lose 4095(commmunism)+25*15~= 4500 units
So T turns from now we have at least
270*T-4500
Merc: assuming 30 cities on average, and Rep from 8 turns from now:
3.25(average gain from a spec in next 8 turns. note that more than half of our cities doesn't have library)*30 for 8 turns
later 7.5*30 beakers every turn after that.
So after 8 turns have passed, gain function looks like this:
3.25*30*8+(T-8)*7.5*30=
780+(T-8)*225
A quick calculation shows that we would need 158 turns to even out. So yes, with this many cities, and low workshop count and low maintenance cost due to Rathauses and few game time left, and no foreign trade routes in communism, it looks like
merc is better.
Pretty much everything is against running communism in this game, except maybe our empire size and the land's workshop potential.

Blub, you were pretty convincing, and now I'm sure that you are right. It's clear that the loss of the wonders is not significant. Communism tech cost is significant, the nerfed maintenance, the absence of foreign trade, and the limited game time.
EDIT: I forgot that we won't lose the specs in the communism variant until we do the switch. Well,
it's about 780 beakers difference, again not significant.