You can get the trunk lid on ebay for about $300-350. Installation should be around $200...not sure since I installed my wing spoiler at the same time. Cheaper would be to find someone with a LX / EX who wants to swap with you so they get the lip spoiler for free.
I averaged 3K a year on my IS350. Now only has a little over 30K after 11 years but I made sure to do an oil change once a year at the dealer. Figure paying $70 a year to maintain the car ain't too much.