This is about 25% more expensive than the UK price, so not way out-of-line. You might find the G4 somewhat cheaper in practice. The G5 transmitters have a 3 month life and have a built in timer, so you can't use them for much longer. The G4 transmitters (which are the same price in the UK), have a 6 month guaranteed life, but you can continue to use them until the batteries die. I got 12 months out of my last one.
As noted by others, although the sensors have a 7 day life, almost everyone extends this typically to 2-3 weeks using a simple trick. When the sensor reaches 7 days, it turns off. Rather than inserting a new sensor you just leave the old one in and tell the receiver, you are starting a new sensor. Works a treat. If you decide to go ahead,, check out this site for tips on how to tape up your sensor to extend its life.
Also, with the G5, do you need the receiver or can you just use an iPhone?