Does Game theory consider opportunity cost?
Recall that Oppurtunity Cost is referred to the benefit forgone by choosing the other option.

In your example of a single player game, opportunity cost exists since the architect is completely in control of the benefit he receives. In this case the opportunity cost for signing the contract is 0 (and conversely -40 for not signing the contract).

However this is only applicable in a single person game.

In a case where the business owner and Architect were actively negotiating, it would appear that there would not be any practical ramifications for considering opportunity cost because the payoffs change depending on the other player and can only be seen after the fact.

for example consider a single period 2x2 game such that:
$$\left[\begin{matrix}(10,5)&amp;(2,2)\\(2,2)&amp;(5,10)\end{matrix} \right]$$

From the horizontal/vertical player's position his payoffs range from 10,5 and 2. He does not choose with respect to costs, he chooses based on what he thinks the payoffs will be.



To answer the broader question which  you are probably wondering about.
Opportunity cost is only considered when a buy in for a specific game is necessary

You'd have to consider expected value of a game compared to the cost. (however that only provides a fair price when the game is played infinity, you would also have to consider how many times you'd have to play a game and end up with a positive profit) using this formula: 

$$n=\left(\frac{1.96\sigma}{p-\mu}\right)^2$$

I suggest reading up on Martingales and the St. Petersberg Paradox for more info.