As mentioned above the exact CPC is going to vary based on a number of factors.
With a smallish budget you start off with a disadvantage against your larger competitors but this does not mean you cannot compete, you just need to think smart and target intelligently!
In addition to the BMM and call only campaigns mentioned I would suggest looking at even longer tail keywords (for example long term manhattan office space lease - I do not know the market but you get the idea, you can focus on areas of manhattan, types of office spaces etc). The objective is to focus both your keywords and your ad groups very tightly and hopefully gain a competitive advantage against your larger competitors who might not be doing such tight targeting because they have the larger budget and therefore do not put in the extra effort (it happens more often than you might think!).
Tight targeting like this should allow you to get high quality scores on your keywords (hopefully reducing cpc) and at the same time help reduce irrelevant or more generic office leasing clicks that will just waste your limited budget. Negative keywords will also be important to avoid the 'shared' and 'short term' type searches, there will probably be a lot of relevant negative keywords!
If you set up your targeting very specifically then you should be able to get better qualified traffic at a better cpc and therefore hopefully a better conversion rate. This is important when you look at your numbers. They look correct if they are based on data you already have from your site, but consider what happens if ANY of those numbers change: If you get 1 contact for every 10 visitors instead of 15, if two out of ten are good leads instead of one out of ten, if your average cpc is 15 dollars instead of 20. Changing any of those numbers makes the situation look better, changing all of them makes it look A LOT better! You will only get real data once you try - just target intelligently to increase your chances of success!