Because these are some of the most complicated and technically advanced pieces of technology that we have ever created.
To communicate with them we have a worldwide array of massive satellite dishes (Deep Space Network) which needs to be operated part of the cost is operating that. Then there is the scientist using the data. These are some of the greatest scientists in the world they are getting paid well enough. Then there is the engineers which make sure the spacecraft operates correctly which are expensive good engineers are expensive.and obviously all the other costs associated with it like facility, technology electricity etc.
To communicate with them we have a worldwide array of massive satellite dishes (Deep Space Network) which needs to be operated part of the cost is operating that. Then there is the scientist using the data. These are some of the greatest scientists in the world they are getting paid well enough. Then there is the engineers which make sure the spacecraft operates correctly which are expensive good engineers are expensive.and obviously all the other costs associated with it like facility, technology electricity etc.