I have been tempted to use fishing line instead of tippet, but when I look critically at the sizes, I always decide to go with the (smaller diameter) tippet.
However, I refuse to pay the $15-$18 retail price for a 25 m spool of fluorocarbon. I look for sales on fluorocarbon tippet (which really does not degrade over time like nylon tippet) and stock up when I find a sale. For example, I am expecting to receive 4 spools of Seaguar fluoro tippet today that I purchased at Sierra Trading Post for less than $5 per spool. Another approach is to use the coupons that Orvis sends out ($25 off on a $50 purchase) to buy $50 of tippet for $25, which comes to about $6 per spool.