wow you guys are funny. read the question again. "...how much would a 60 watt bulb cost per month?" actually, it really depends on how much the hardware store sells it for and how many you buy per month. hehe. sometimes, 60 watt bulbs cost the same as 45 watt bulbs. the answer in this case is speculative if you really think about it because it requires you to know how much a 60 watt bulb costs.
now, considering that the question was asked in such the manner that it apparently received all of the above answers ($4), it is still impossible to know what the REAL cost is due to some missing facts. 1) was the 60 watt bulb used exactly the same amount (in duration) the 45 watt bulb was used? you guys don't know this so you automatically ASSUMED that they both were used exactly the same duration. 2) was the 60 watt bulb a bulb that is equivalent in luminence rating to a 60 watt bulb (such as a 7 watt neon bulb) or was it actually an incandescent 60 watt bulb? 3) does the lamp (or light fixture) have a built in dimmer? if so, no matter if the bulb is 60 watts or 45 watts, it will still cost the same depending on the impedence of the dimmer. hehe. so anyway, my point is that this question is not very definite; therefore, it can not receive a definite answer. $4 is a POSSIBLE answer but is not necessarily the ONLY correct answer.
furthermore, in order to receive all the above answers, i think this question should have been written as "if BULB-A (45 watt incandescent light bulb) costs $3 per month in electricity usage fee after having been left on for exactly X amount of time while installed on a light fixture that has no dimmer implemented, then how much would it have cost if BULB-B (a 60 watt incandescent light bulb) had been in BULB-A's place instead?" :)
i'll admit that i'm not smart; but it doesn't take a genius to figure this one out! lol