When will it be economical to harvest the resources you need in space rather than lift them from the surface?
Well, that of course depends on several factors:

1. the cost per kilo of lifting the resource (obviously)
2. the cost per kilo of bringing the resource in from where you found it in space
3. the cost/value of the resource.
   The first two are pretty obvious. If it costs 5000 dollar per kilo to lift a chunk of whatever from the surface, and 500 dollar to bring that same chunk over from the asteroid belt, that's a cost saving of 4500 dollar per kilo.
   
   But that doesn't tell the whole story. The cost of extracting that material on the surface is likely a lot lower than the cost of extracting it in say the asteroid belt, at least initially (only once the initial investment has been paid off will you possibly be competitive, and that might take a while).
   
   Also keep in mind that if large deposits of something that's very expensive on earth now are found out in space, the value of that material is likely to plummet, unless the people finding it are very careful in piping it into the economy only in small amounts.
   Say the price of platinum is now 10000 dollar per kilo (just a figure, no clue about the actual price), with a world wide production of 10 tons per year.
   Now say I find me an asteroid that has 500 tons of the stuff in it, and I decide to mine the thing. I can mine it at a rate of 100 tons a year. If I dump all that 100 tons a year in the world economy, the price of platinum is going to drop and drop hard. Instead of the 10000 dollars per kilo I expect to get I might after a few days of it only get 500 dollars per kilo.
   So I have to make sure I feed that platinum in in small amounts only, say 10 tons a year, which might not even cover the cost of installing and maintaining my mining operation...
   
   All this assuming there's a healthy on-orbit society already from which to draw manpower and support infrastructure. If you need to build that as well, the cost goes up far higher, making the whole venture into a loss leader from the outset (which btw is one reason there's never yet been put in place things like solar power stations in orbit, the initial investment to get the permanent settlements in space up that are needed to maintain and support such industry is too high for interested parties to bear).

When will it be economical to harvest the resources you need in space rather than lift them from the surface?

Well, that of course depends on several factors:

1. The cost per kilo of lifting the resource (obviously)
2. The cost per kilo of bringing the resource in from where you found it in space
3. The cost/value of the resource.

The first two are pretty obvious. If it costs \$5000 per kilo to lift a chunk of whatever from the surface, and \$500 to bring that same chunk over from the asteroid belt, that's a cost saving of \$4500 per kilo.

But that doesn't tell the whole story. The cost of extracting that material on the surface is likely a lot lower than the cost of extracting it in say the asteroid belt, at least initially (only once the initial investment has been paid off will you possibly be competitive, and that might take a while).

Also keep in mind that if large deposits of something that's very expensive on earth now are found out in space, the value of that material is likely to plummet, unless the people finding it are very careful in piping it into the economy only in small amounts.

Say the price of platinum is now \$10000 per kilo (just a figure, no clue about the actual price), with a world wide production of 10 tons per year.

Now say I find me an asteroid that has 500 tons of the stuff in it, and I decide to mine the thing. I can mine it at a rate of 100 tons a year. If I dump all that 100 tons a year in the world economy, the price of platinum is going to drop and drop hard. Instead of the \$10000 per kilo I expect to get I might after a few days of it only get \$500 per kilo.

So I have to make sure I feed that platinum in in small amounts only, say 10 tons a year, which might not even cover the cost of installing and maintaining my mining operation...

All this assuming there's a healthy on-orbit society already from which to draw manpower and support infrastructure. If you need to build that as well, the cost goes up far higher, making the whole venture into a loss leader from the outset (which btw is one reason there's never yet been put in place things like solar power stations in orbit, the initial investment to get the permanent settlements in space up that are needed to maintain and support such industry is too high for interested parties to bear).