For muscle building and muscle optimization, I use the following:
2.2lb bulksupplements creatine ($43.96, 200 servings) = ~$0.22 per serving
(deal currently on amazon, 10% off two) 2x bulksupplements creatine ($79.13, 400 servings) = ~$0.20 per serving
1.31lb optimum nutrition creatine powder ($52.72, 120 servings) = ~$0.44 per serving
2.2lb myprotein creatine monohydrate ($76.99, 200 servings) = ~$0.38 per serving
2.2lb nutricost creapure creatine monohydrate ($57.95, 200 servings) = ~$0.29 per serving
2.2lb nutricost micronized creatine monohydrate ($44.95, 200 servings = $0.22 per serving
(deal currently on amazon, 10% off two) 2x nutricost micronized creatine monohydrate ($80.91, 400 servings) = $0.20 per serving
---------creatine for horses (wink wink)--------------
creatine 5k ($699.99, 2270 servings) = ~$0.31 per serving
Creatine will tend to increase water retention. To a certain extent your muscles will get bigger and plumper and you may put on weight. For example, I weight 175 or so and put on up to 5 lbs when starting creatine again recently. I also started lifting seriously at the same time though so some of that weight was probably also muscle. Anyway, this is water weight, so it's not bad, but it if you're checking your weight regularly it may discourage you if you're not expecting it. The general consensus is to take 5g a day of a reputable but non-fancy brand (i.e. you don't need Dr. Insano's Mega Bro Gainz Creatine and Synthetic Steroids Grape Blast!). I use this.
Serious bodybuilders will stop using creatine many works before a competition to allow their muscles to get rid of their excess water and look more cut but that's obviously beyond the scope of your average person looking to get into better shape.
Here's two helpful links with more details:
http://examine.com/supplements/Creatine/ http://www.bodybuilding.com/fun/animalpak22.htm
Amazon is a great source for well-priced supps.
Lol no, they aren't affiliate links.
They're still dirty links. Clean them up when you share them.
Vs
http://www.amazon.com/Optimum-Nutrition-Creatine-Powder-Unflavored/dp/B002DYIZEO/
But dem referral links...
Edit: as an example, here's a link they posted: http://www.amazon.com/Optimum-Nutrition-Creatine-Powder-Unflavored/dp/B002DYIZEO/ref=sr_1_1_s_it?s=hpc&ie=UTF8&qid=1459917518&sr=1-1&keywords=creatine
And here's the link without the extras: http://www.amazon.com/Optimum-Nutrition-Creatine-Powder-Unflavored/dp/B002DYIZEO/