Simple Guide to US postage prices

US First-Class Mail Prices (effective 11 May 2009)

Weight		Normal Envelope	Large Envelope
< 1 ounce	< 28 g	$0.44	$0.88
1 - 2 ounces	29 - 57 g	$0.61	$1.05
2 - 3 ounces	58 - 85 g	$0.78	$1.22
3 - 3½ ounces	86 - 100 g	$0.95	$1.39
3½ - 4 ounces	100 - 114 g	Same Rate as Large	$1.39
4 - 5 ounces	115 - 142 g		$1.56
5 - 6 ounces	142 - 170 g		$1.73
6 - 7 ounces	171 -  199 g		$1.90
7 - 8  ounces	200 - 228 g		$2.07
8 - 9 ounces	228 - 256 g		$2.24
9 - 10 ounces	257 - 284 g		$2.41
10 - 11 ounces	285 - 313 g		$2.58
11 - 12 ounces	314 - 342 g		$2.75
12 - 13 ounces	343 - 370 g		$2.92

Forever stamps are always worth the same as 1 ounce of first class postage.  Currently they are the same as a 44-cent stamp.  So two would be worth 88-cents of postage.