It's difficult for content agencies to be upfront about pricing for the same reason it's difficult for development agencies.

(I'm a freelancer in the same space as this service):

Much like development, cost depends on the scope and complexity of the project. It can't be measured as a multiple of a single metric that can easily be displayed on a price grid.

Another reason to be coy about pricing is so that they can charge different rates to clients. Most content creators wouldn't charge Google the same as they'd charge bootstrapped new tech startup.

It's also possible they simply aren't interested in dealing with excessively price-conscious clients. The "if there are no prices, it's too expensive" approach isn't accidental. It's done on purpose to avoid having to deal with the lower end of the market. It sends a strong signal to the wrong sort of client.

I do something similar in freelancing work: I'm not interested in working for people who want me to write blog posts for $40 a pop, so I price myself way too high for this sector of the market. I also don't display prices because I don't have to — referrals and reputation are sufficient.