Well it is certainly easier, although I don't know about cheaper. I usually make this when strawberries are in season and selling for dirt cheap. I think the most I've paid for the 4lbs of strawberries I've needed for this beer is around $6, and usually it is $4 or less.
Those little bottles of flavoring/extract are not particularly cheap.
I personally don't use the extract, *especially* the strawberry extract, because it has a peculiar flavor to it that tastes wholly artificial to me. You can taste it in e.g. Pete's Wicked Strawberry Blonde. That is why I came up with this recipe in the first place. However, my tastes are not the same as everyone else's, so if you want to do flavoring additions go for it. As to how much...my advice is to pull yourself maybe a 12 oz sample (be sure to measure the amount carefully...use a measuring glass to hold your sample) from the primary and try adding extract to it with a dropper for which you know the amount in each drop. Then you can slowly add drops, mix, and taste it until you get to the level of strawberry flavoring you like. Count how many drops you added, and figure out the ratio you need for the full batch.