The new agony infusions do a great job of showing the difference between cost and value. A one-unit increase in value doubles the cost.
They work like this: +1 agony (resistance) infusions drop. You can use one in an infusion slot to give an item +1 AR, or you can combine it with an identical infusion to increase its number. Two +1s get you a +2, two +2s get you a +3, and so on. It can be fun in guild chat to see someone who has never seen how quickly doubling numbers grow. You need 16 +1s to get a +5 and 512 to get a +10.
Assuming a trading post cost of 20s per infusion (the last time I looked), a +5 infusion costs about 3.5g, including the cost of combining them. A +10 infusion would cost 102.4g, which rounds off nicely to 110g when you add another 7.6 for all those combinations. A +20 infusion would cost nearly 8000 gold ($1,876 at the current gem exchange rate) just for the combination costs and would also need more than a half-million +1 infusions.