If you just want a inexpensive but very strong snatch block check out the
http://garhauermarine.com/catalog_process.cfm?cid=26 ones. They have a 5000 lb working load one for 150 dollars directly from them. The model number is s70.
I was looking for one to use for my storm drogue for heaving to in rough weather and couldn't find anything thing of a size to handle 3/4 inch line unless I was spending 600 dollars or so. It was depressing. I couldn't believe the price for the Garhauer one it was so low compared to the other brands. I ordered two of them and was very impressed with the quality. When you hold them in your hand they just have that solid quality machined feel to them. These are big blocks and weigh a little over 2 lbs at 36 oz.
The prior mentioned posts suggestions would probably weigh much less though.