SavingForTravel have a great tool I came across a while ago. The Travel Budget Calculator provides an estimate of travel costs for most countries in the world (sadly Djibouti is not covered). Simply select the destination(s) and the number of weeks you intend to stay in each country. The site will estimate (in Pounds Sterling) the cost for a ‘no-frills budget traveller staying in hostels or guesthouses, eating as cheaply as possible, travelling an average amount internally and seeing the typical sites’. Even if this description doen’t match the way you intend to travel the site can give you a quick snapshot of the likely cost of a trip or the comparative cost of two or more destinations.
I’ll need £420 for three weeks in Kazakhstan, but £378 for Kyrgyzstan, apparently. Jagshemash!