I photographed this at Nairobi station on 31 October 2011. I can't make any claims for its accuracy - and the landline numbers 2044476 and 2044479 appear to be two digits too short (they are probably actually 020/44476xx and 020/44479xx but almost certainly not working anyway) - but it's all the information that's available. The twice weekly Nairobi Mombasa Nairobi service is definitely running at present, with these prices.
2nd class: Ksh1150
1st class: Ksh2170
Bedding: Ksh320 (automatically included with ticket, not sure if you can avoid paying)
Breakfast: Ksh470 (optional)
Dinner: Ksh700 (optional)
Incidentally, the Nairobi Kisumu Nairobi service is currently suspended once again (they told me on 31 October that the last train from Kisumu was departing that evening).
2nd class: Ksh1150
1st class: Ksh2170
Bedding: Ksh320 (automatically included with ticket, not sure if you can avoid paying)
Breakfast: Ksh470 (optional)
Dinner: Ksh700 (optional)
Incidentally, the Nairobi Kisumu Nairobi service is currently suspended once again (they told me on 31 October that the last train from Kisumu was departing that evening).