I have used Peaceland's services for my last three visits to Nepal. Their fares seem to be quiet competitive. It does not usually matter when in May you are traveling. Normally, airlines consider April and May as "shoulder season", and June-July as "high season", with higher fares for the latter two months. Fares seem to be the highest in August as well as from mid December to early January while they seem to be the lowest in Jan-Feb and Sep-Oct. Hope this helps.