The best and most cost-effective marketing mix would, first, include direct mail. Send out a professionally designed and written piece to target high income families within a 10 mile radius of your zip code. Secondly, network: Give out grand opening discount coupons to various business groups of the area in person. Then put print advertising in your local daily and/or weekly papers (assess which ones are well-read by people falling into an upscale demographic, as they will be your prime prospects.)
Depending on where your spa is located, you could also purchase train station platform advertising. Choose stations that have a decent ridership of people that have at least an average yearly income of $80,000 (the people who sell this kind of ad space can provide you with those stats). Last but not least - invest in some good PR as editorial coverage is worth gold!
Best of luck with your new venture!