USD $40 US Dollars is the price for Lao Visa on Arrival (Lao VoA) and the cash currency should be pristine (clean, "new") banknotes, because the Thai Baht equivalent price will not be determined by current exchange rates, and, honestly, up to each individual officer whether they will take Thai Baht or other currency instead of pristine US Dollars, with an exception for China and Vietnam where their stated Lao VoA price is USD $20
Applications from outside of Belgium or Luxembourg as well as any pending visa applications from foreign nationals who are currently in Thailand will be automatically denied.