The heavier the gauge of twine the less nets you'll get out of a 500g or 1kg cog - and heavier gauge is generally more expensive too. Obviously, mesh size has a bearing on how many nets you'll get out of a cog, as does the width/length of the net.
In short the answer to your question is the same as that to 'how long's a piece of string, nylon, hemp'!
My advice: get a cob of the cheapest you can find initially (I'd get some cheap 4-ply hemp off the guys on here) to get some practice in, then try other types of twine. I use 5-ply hemp (but that's not to say it's the best, just my prefere