Welcome, janie! :waving:
FYI: Merino is a type of wool, not a weight. It’s supposed to be the softest type of wool.
http://en.wikipedia.org/wiki/Merino warning, cute pics of little sheeps!
In the pattern you wrote about, the 73 yards/50 grams tells you how many yards are in one gram of the model poncho’s yarn. To find this, just divide 73 by 50 = 1.46 yards per gram. If I remember right from this pattern, that’s just telling you the weight of the yarn that was used in the model. Don’t worry about finding it in the same amounts per skein/ball/hank/whatever. Just find a yarn that’s the same weight per yard. The types of yarn weights, from lightest to heaviest, are : lace, baby (a.k.a. fingering), sport, DK (double knitting - I don’t know why it’s called that), worsted, bulky, and super bulky. Various yarns are labeled in various ways, but this is the standard. Worsted is the most common weight of yarn for hand knitting.
When you’re knitting a garment, you want to look at the weight of the yarn, not necessarily the fiber content. If it’s worsted weight, you can substitute other (cheaper!) worsted weight yarns for the yarn called for. Yarndex.com will show you different yarns that you can substitute for each other. It’s always helpful to knit something for the first time in a really cheap yarn, like a wool-acrylic blend, especially if you’re a new knitter.
Different fibers (wool, cotton, acrylic, etc.) will make your finished garment feel and look a bit different, but the size should be the same if your yarn weights are the same.
Also, if you want some relatively nice but cheap yarn, if you have a Michael’s craft store in your area (www.michaels.com), they’re having a huge yarn sale this week! You can get some 100% merino wool (Patons Classic Merino) for $4.
Oh, another thing to keep in mind about wool: If you wash it in the washing machine and dry it in the dryer, it will felt and become Barbie-doll-sized. You can get superwash wool, which can be machine washed and dried, but be sure to READ THE LABELS! Many knitters have been heartbroken over accidentally felted handknits. (not that I would know from personal experience, or anything…)