A lot of the expense from saltwater tanks comes from the lights & filtration equipment. For a mantis, you can use regular Normal Output flourescent lights like come with hoods at PetSmart and they do not require fancy filtration.
A small mantis could be kept in a stock 3g eclipse tank with substrate & rocks added. Then your regularly-occuring costs are going to be for water and for food. As folks noted, frozen food with an occasional live treat seems to be pretty adequate and also not terribly expensive at ~$10-20/ month, depending on prices in your area & how much live food you use.
Good luck,
C