The costs involved in doing what you want to do would probably be in line with the costs of simply building an addition. In BOTH scenarios, the problem I see is that you might price yourself out of the nieghborhood and not recover your investment.
There are 2 things I suggest you do. There are a number of WEB sites that provide Multiple Listing Services (MLS). Find homes similiar to yours that are splits, that have (and/or don't have) an addition, that have (and/or don't have) a dug-out crawlspace. The 2nd thing to do is get an estimate for an addition AND an estimate to have your crawlspce dug out and finished. (The key is finished.) With all this information, you should be able to determine if it's money well spent. It's not that hard to do; it will just take some time to get the information together.
I would say, in advance of what little I know about homes in your area, that the first option would be to build an addition. The 2nd alternative would be to move to a house that either has what 'space' you want, or has the 'potential space' you want w/o having to dig out a crawspace. Now, on the other hand, if you're gonna stay in your home for, say, 15+ years, you might recover your investment in your split. That's a big 'BUT' that has a lot of variables. Perhaps others have their $.02 to add.