Can this product be used in a built in spa with tile and plaster?

I presume so, the effect of using this product is microscopic particles clump together and form a paste-like substance around the tub at the water line, which leaves the water clear. You will need to wipe off the spa at the water line to "finish" the process. If your spa has a porous surface at the water line, it would be a mess, but ceramic tile shouldn't be too hard to clean. Hope that helps.

How quickly can you tell that it is working? Does it take minutes, or hours, or a day?

Depends on the condition of your water when you place the product in the water. It will work in minutes if the water is not too bad. Should be used on conjunction with the rest of the Leisure Time products.

Does any one know why did the price go up from 13.23 to 15.52 in ~7 months?

The advantage to online selling is you can change prices on the fly based on inventory. If you are low on inventory and you may sell out before you get more, you’d be STUPID not to raise the price to slow down sales and improve ROI. It is NOT “greed”, it’s common sense retailing.

How often do you use this product? We use our Hot Tube 2-3 times a week Thanks

This can be added as often as needed. There is no set regiment for this chemical. It's one of the items used to help clear water. You use your spa 2-3 times a week. It also depends on how many bathers use it. It helps with removing smaller particles in the water. This is not a preventive chemical rather it's used when you need to make the water clearer. If you don't have problems with cloudy water then you may never need to use it. Considering though all the chemicals and minerals in water supplies you most likely will use this product. You can also do some comparative shopping on the internet. There are other products out there besides Leisure Time. The Spa Depot specializes in spa's and hot tubs. Some of their products are comparatively priced. I buy both from Amazon and Spa Depot.