First thing you need to do is tell us a little more about this space your working with..what you've already said is pretty basic in terms of details. What I mean is there's a difference between a brick wall with dry wall fixed to it and a cavity wall, compared to a stud partition dry wall. Each kind would need to be treated differently in order to get the level of soundproofing you're talking about..which means the costs involved are going to be different, too.
You say you've leased this space..it might be worth your while in the long term to relocate to a basement-type setting, as a basement will usually cut your soundproofing costs for obvious reasons.
Also something to keep in mind is the floors and ceiling..are they concrete, timber or tiles? Is it a false ceiling..likewise, is there a crawlspace under the floor? All these things will have a big impact on your budget..and even if you had to cut your losses and relocate, it might be worth doing so if you find the right place that will offer a good level of natural soundproofing.
No matter what you do..stay put or move..it's going to cost you a lot to get the level of SP you're aiming for..even though you can use everyday things like furniture and bedding to dull down some sound, you're still going to have to splash out a lot.
My suggestion would be that you take a good look at this location you mentioned and see just what you have to work with before you start asking how much it's going to cost, as you can't really get the answer to that until you really know what you are dealing with.
[EDIT] You should also check out [Only registered and activated users can see links. Click here to register]
for a more detailed account of what's what about soundproofing.
Best of luck.