Put 100 tons of water in a orbit which last more than 5 years, and I would predict that within 30 years you will have powersats beaming power to Earth.
Or mine 100 tonnes of water in space and I predict that within 20 years are will have powersats beaming power to Earth.
It would require about 5 hvy launchers which are already developed and are currently launching satellites into space. So the govt could simply buy about 5 launches at something around 100 million per launch and get 100 tons into an orbit which last 5 or more years. So it cost about 1/2 billion.
There are ways that the govt could get 100 tons into orbit cheaper and better than what I describe above, but that's just details.

The point is that govt pays for these water payloads, then offers to give these payloads to a business which will use the water to make rocket fuel [LH&LOX]. In sense this acts a prize. It's a prize to anyone who can build and launch a gas station that will convert the water into rocket fuel. What this does is begin a market for water in space. There is a lot a water in space. What is needed in space is rocket fuel. If you have a market for water in space [and this doesn't even neccessarily mean mining water in space [yet], but it could be merely shipping water from earth into orbit to a gas station who then buys it, then turns it into rocket fuel and then sells the rocket fuel] you will have a more robust market for electrical power in space. The first gas station might have only about as much solar panels as a large communication satellite and reqiure a fairly long time to process 100 tons of water, but it's reasonable that the market could evolve so that you need to process say 100 tons per month- this takes about 10 times or more solar power than your large communication satellites. With market forces launch prices can drop in price, electrical power in space can drop in price, and of course rocket fuel will drop in price in space. Now, you could fairly easily get to point of say water in orbit costing about $500-1000 per lb, electrical power costing about $10 per kw/hr and rocket fuel costing somewhere around $1500-2000 per lb. Now electrical power at $10 per kw/h is cheap in space, but on Earth electrical power goes around $.05 per kw/h. It's far easier making a powersat which provides electrical power in space and sells it in space without beaming to a location on Earth for a price which is say $1 per kw/h than selling on Earth on for .05 kw/h. But it's possible that say powersat is selling lots of power in space for say $.50 kw/h and to increase the amount power sold you broaden your market to include some of power being beam to earth, for people who for whatever reason need power and will pay the .50 kw/h because compared buying/leasing generator and dragging them to some location and setting it all up, all they need instead is a microwave reciever and get the power they want for as long as they want- say for weeks time period. Or probably your biggest potential customer might been the US miltary- they might buy lots of power for long period of time, and pay lots of money. They can save themselves all the oil tanker runs through hostile terrority. Think about if they air lifting in fuel for power generation they could be easily be paying much more than $.50 per kw/h of electrical energy.
Anyways, as market gets bigger, there is downward pressure on market prices, as prices drop to say $.10 kw/h more customers on Earth will want to get this power- emergence power for cities, peak load, etc.

With market for water in space, you might mine the moon for water, but also you can get water from Asteriods. Say a asteriod has a million tonnes of water [fairly common] if there large market for water and it's selling at 20,000 a tonne [$10 per lb] the asteriod has 20 billion dollars worth of water, which might cost 10 billion to extract giving a profit of 10 billion. Of course you might get this asteriod when water prices are still $100 lb and make 150 billion profit [assuming your costs are higher- 50 billion in total].