Ethiopia is one of Africa's safest places to visit. Crime against tourists is actually very rare. The real dangers are in road travel when collisions are not uncommon, so I would fly when travelling the tourist circuit rather than use the local buses. As for cost of travelling in Ethiopia, it is extremely cheap, unless you aspire to luxurious eating/sleeping when you wil pay the same rates as you would anywhere in the world. Local food is a couple of dollars a meal, and its delicious, guesthouses are seldom more than $30 night.