Even if you had blueprints that said "here's exactly how to build a space telescope" just building it and going through all the proper testing would be quite expensive. Certainly you could just privately fund, say, Johns Hopkins University's APL to build a spacecraft for you, but it would still cost hundreds of millions of dollars and still be a comparatively risky endeavor. More so when you consider that the operations side would run to the millions of dollars a month level, and require a substantial build out of infrastructure.
That sort of thing will happen, but it'll probably take until after launch costs have dropped a fair bit.
I'm not sure which Mars landers you're referring to, I'm not aware of any that were so cheap. The Mars Science Laboratory (Curiosity rover) cost $2.5 billion. The total cost of the MER rovers (Spirit and Opportunity) was $820 million. The Mars Phoenix mission cost $420 million. The Mars Pathfinder mission cost $280 million in 1996 dollars, or $380 million adjusted for inflation. There have been no other successful Mars landers within the last 2 decades.