If you are willing to forgo some low frequencies, such a project doesn't have to be expensive. Another thing, if this is to be used in some outdoor battlefield simulation, only a single speaker may be needed, not two channel, which will alleviate costs. However, the speaker will need to be outdoor grade, and that will not be easy.
Such a speaker will need caution for use though. Exposure to 130 dB + can very quickly cause permanent hearing loss.