I believe that the problem here is perception. All ministires have costs. TV/radio ministers such as Charles Stanley, John Hage, Joel Osteen and the like have huge organizations that must be run, and funded, in order to further the Word of God all over the world. All media-based ministries, including this one, are funded by donor gifts which are given freely by believers who support their mission(s) to bring the Gospel to as many souls as possible.
If this web site we are discussing has set a direct monetary value on prayer; e.g., "for $5, $10, whatever, we will pray for you", that is absolutely wrong. However, if they are soliciting donations to help offset the costs of operating their ministry, I believe that this is perfectly acceptable. After all, that is probably their only source of revenue.
God bless.