Because all of the technology and machinery necessary to go to the moon was designed and built. The massive and massively expensive Saturn V rockets(many of them) were designed, built and launched to witnesses. The LEM, CSM, suites, computers, and etc were all built. The simulators were built and simulations run. Etc and etc and etc; hundreds of thousands of people invovled.
So, pretty much all the cost of going to the Moon and now add on the cost of faking it and maintaining that lie for 50 years.
And they would know that "maintaining that lie for N years" would prove useless as soon as the powerful cameras in lunar orbit would photograph the landing sites. Who would participate in such a lie knowing 100% it will be exposed, eventually...
So, pretty much all the cost of going to the Moon and now add on the cost of faking it and maintaining that lie for 50 years.