So, if I understand this correctly, giving several unique wonders of this type to a civilization that otherwise would not generate very much Commerce would be a way to give it some boost, correct? I mean, your base GPT would have to be fairly high to begin with before you reach the +50 GPT ceiling.
That is if a civilization without these wonders has a GPT of 100. So, one wonder with this effect is an additional 5 GPT. Therefore, ten wonders of this type would be the full 50 GPT - again assuming a 100 base GPT.
So a much smaller, much poorer, more infrastructural lacking civilization might bring in, say, 10 GPT base. Given ten wonders of this type, its GPT would only be increased by 25 - but that 25 would be a very significant increase.
Unless anyone else knows of a better way to reach this goal?