Help with amount of yarn needed for pattern

[RobinOK]

I'm fairly new to knitting. I'm going to knit a throw made up of 20 squares, finished measurements 48 x 60 inches. The wool according to the pattern is worsted weight yarn 200 yds/100g per ball, size 9 needles, gauge 20 sts = 4 inches, 13 balls in total. However I've spotted this gorgeous super chunky yarn, 70 yds/100g per ball, gauge 7 sts = 4 inches. I don't know how many balls of this wool I need, so I rang a woolshop today and was told by this guy that rather than needing less wool (as I had thought, I was under the impression that the chunkier the yarn, the less you would need, but I could be wrong!) I would need a lot more, in actual fact I would need about 36 balls, which would set me back a colossal amount of money ! Is he right?