Considering you only need to get the Commerce opener to build the Big Ben, do you feel that it is optimal to spend 2 Social Policies simply to acquire the benefits of mercantilism on purchasing spaceship parts? Do you not feel there is a more optimal use for those two policies?
Big Ben by itself is not that strong. Because of the way the purchasing bonuses are calculated, it is very efficient to add mercantilism. Big Ben + Mercantilism get added up (for a 40% discount) but this is compounded with the discounts for purchasing units/buildings from Autocracy and Order (33% discount).
The purchase cost is calculated by multiplying the modifiers (initial cost * multipliers), so for Big Ben alone the multiplier would be 0.85, while if adding mercantilism, we will have 0.6.
If we add the bonuses for purchasing units/buildings, they will be compounded, so 0.6 * 0.67 for Big Ben with mercantilism, or 0.85 * 0.67 for Big Ben alone. So you will not have a 77% or 48% reduction in purchasing things, like one might expect, but a 59.80% or a 43.05% reduction.
A common strategy for Freedom Science wins, is to get Big Ben + Mercantilism (maybe even finish Commerce later, because if you have enough faith, you can buy a merchant, and get double gold from it, which can help with space parts, also the extra happiness may give a golden age, which will be 50% longer with Universal Suffrage). Of course Freedom does not have an extra discount for purchasing things, so the maximum you get is 40%, but this will help you significantly to buy the space parts, and in most cases you don't even need to hard build any.
Now you are saying that Mercantilism is a waste because you need to get Mercenary Army for it. The thing is Mercenary Army is not useless. Landsknechts are cheap and you can have them be your primary melee unit, if you get the policy early enough, especially if you beeline techs on the top part of the tree (meaning you don't have steel). They can pillage and fortify (plus heal) on the same turn, so they have a greater chance to survive (compared to pikemen), and will give you some gold over time especially with the double pillage from cities. With Big Ben and Mercantilism, they are so cheap that with freedom you can rush buy them and gift them to city states for 20 influence.
Plus I see that the fact that Mercantilism gives you science, 1 per Market, Mint, Bank and Stock Exchange is ignored. You will most likely have markets, mints are situational, but later on you will add banks, and even Stock Exchanges in some cases. So this bonus is roughly the same as having an extra citizen in all of your cities (like half the bonus of a CN tower, only in medieval era).
Of course depending on situation, there can be a better use for 2 SPs but getting mercantilism once you open commerce is pretty solid.