Here's a good technique for weakening the main force of an enemy, before he gets the chance to really get a force built. Use whatever build techniques you are comfortable with, but push to get Shuttles as fast as possible. Here's the strategy: Take in 2 Shuttles and load them with these following units:
Shuttle 1: 2 Goliaths

Shuttle 2: 5 SCV's and 3 Marines

Land the SCV Shuttle near an outside area of your enemies perimeter (use scanning to determine this). Have 1 SCV immediately build a Bunker, and the other 4 SCV's build Missile Turrets at the 4 Corners of the Bunker. Once the Bunker is built, store the Marines in it quickly, and ONLY once the Missile Turrets are built, land the Goliaths on the furthest side of the Bunker away from the enemy. Now you have a very good Anit-Aircraft site near the enemy base, but if the plan fails while the Turrets are being built, at least you can save 2 dropships and 2 Goliaths. The best strategist will think you're trying to build a base near theirs, and might try to bomb you out of the water, but I like to use this strategy to do one of two things: A. Build a few Barracks near the AA site once established, or
B. When I decide to engage the enemy with my Wraiths, draw them away from the battle once the enemy's ships arrive, and take them over the AA site. Enemy ships usually drop from the skies when 4 Turrets, 2 Goliaths, and 3 Marines combine to squash them. And that's when you can utilize the Ground attack, when the enemy Air Force is elliminated. This strategy is not too expensive and is quick to pull off. Try it and see.

CreepColony's Opinion: This may not be an expensive strategy, but it is time consuming. It'd be easier just to build turrets.