Every dealer is different. Really, it's mutually beneficial. You get to try a machine without a long term commitment and build equity, while the dealer gets your butt in his machine. Generally RPO rates are higher than straight rental rates. This is because RPO machines are not generally in a capitalized fleet with a long term depreciation schedule. Additionally, we prefer to be responsible for maintenance while the machine is on RPO (another cost). This insures that it was well cared for should you decide to return it. Every dealer is different.
The amount of rent to apply to the purchase price varies by dealer and machine as well. The first thing you need to do is negotiate the sale price, then ask about an RPO option. Different dealers will go for different periods of time. Keep in mind that dealers are not banks and they don't like to carry RPOs for any significant amount of time. We apply rental credit on a declining basis, the longer that you have a machine, the lower % of rent to apply.
Feel free to PM me with specific questions.