This is a tough one, L.. There's so much going on here: power/control issues, fairness, the community nature of family income, disparate incomes, pride, fairness, retaliation, and dishonesty/deception… Oh, my!
Have you talked about goals? Can you project a goal that you feel is achievable and reasonable for the investment? Without a clear goal, what you are contemplating is a huge outlay of cash that might end up achieving very little, or results that don't hold when your program is finished. Are you expecting to achieve a specific weight loss, life-style changes, increased fitness that will contribute to your own AND your family's well-being, improved self-image, an improved love-life, clothes that fit better or a new wardrobe?
I think you can see that some of those goals might be important to you, but create uncertainty or discomfort in your husband. So you would do well to tell him goals that matter to him. And convince him that this program would be a jump-start that will make it possible to maintain those goals at lower cost into the future.
I'm in my 60's, and have engaged in many, many major life-style campaigns. Some have been free, or the cost of a book, and many have conferred tremendous benefit. But ALL of them are temporary unless I have the willingness and ABILITY to continue whatever effort or self-control is required. I have been tremendously fit. I have been overweight and weak. Life continually throws wrenches into my plans; an illness or injury, a time-consuming family obligation, menopause with metabolism changes, loss and depression, you name it. Something is always waiting to take away my dedication to any particular plan of action.
These challenges are there whether my program was self-motivated and free, or whether I had invested thousands of dollars (well, I've never had that kind of money, but $100 would have been a huge, obligating investment for me that might still have lost all value a year later).
So I can sure relate to your husband's reservations. I hope you do not choose to deceive him if your marriage is important to you – that could be the beginning of the end. It sounds like you were on the receiving end of a very good sales presentation. That makes the plan your gym is offering sound very attractive. But it may not be the best, or the only, option you have for meeting your goals.