--- UPDATED ---
now in html and javascript: Link

Desktop Tower Defense has become a bit of an obsession lately. It is a game of strategy, timing, and now: statistical analysis. If you haven't played it, I suggest that you avoid it like the plague. If you're hopelessly entangled with it, then continue reading.

To determine the best use of gold I tossed together a webpage with information about the Creeps and Towers.

• Creeps: type, health, speed, value, count, total gold per level, and total gold to that point
• Towers: Cost at level, upgrade cost, sell value, damage, range, speed.

I also included a "Quick" and "Advanced" Tower Calculator. It will help you determine how many of a particular kind of tower you can buy at a particular level.

Links:
The Sheet (now in html and Javascript): Link
Game: Desktop Tower Defense