Right click on Fm 2008 icon - Select Run Data Editor- File :Load Database - It then gives you the option of what Database to load, in your case "Default" double click it and it will load.

Click on "Clubs", and in the filter section type which club you wish it to search for, once its found it double click, for your club go to the "Finances" section there you can change the transfer budget.

Save the new Database, with a new name. You cant over wright the Default Database so don't worry about losing that. Remember when you load the game to play it you need to select your new database.

If you have any problems, add details to your question and Ill get back to you.