can I extend my open loft?
If you want it badly enough, it's probably possible. Whether it's practical or cost effective is another matter. I don't think anyone could possibly give you any idea of the cost without looking at what you have and what you want. Costs are related to the amount of work involved, local labor and material costs, and the level of finish you want. 

A new floor must be supported by load-bearing walls. If you already have suitable walls in the right place, this might not be a problem. If you don't, you've got to build some, which might mean changing the downstairs and adding to the foundations. 

What are you going to use the space for? There are code requirements about ceiling height, size and placement of windows (especially in a bedroom) and so forth. It may be necessary to run a new electrical circuit from the breaker box. There may be local regulations that say that if you spend more than a certain percentage of the home's existing value on repairs or remodeling you have to bring the rest of the house up to current code. If you have a septic system you may be required to upgrade it if you add a bedroom. 

If you really want to do this you would be best off talking to a local builder who can give you advice specific to your situation.