Hey Andreas,
the Rongai route is fairly easy, comparable with the Cocacola (marangu) route, but without the huts.
Prices are same as Machame route for same number of days.
It is quite dry on that side, but you might encounter some elephants! Have not climbed it myself yet, but heard it is quite nice, especially because itīs really quiet (still). Last day is same as the cocacola route though.
Itīs no problem you start form teh Kenyan side as you actually do not leave Tanzania! You travel alongside the border, but never cross it!
Let me know if you want to try this route and I will create a trip for you.
Thanks and keep climbing,
Harry
Ps: a route diagram of the Rongai route as well as all other Kilimanjaro routes can be found on the routes page:
http://7summits.com/kilimanjaro/rongai.phpPrices are to be found on the trips page:
http://7summits.com/kilimanjaro/trips.html