This really depends on the profit margin. it makes sense for a company to use some of the profit margin to ensure repeat customers. How much makes sense to use depends on how much it increases sales. For example, if you spend 10% of the profit margin and that increases your sales by 15%, you’re making 3.5% more profit (0.9*1.15 = 1.035).
Of course, if they can increase their profits by increasing the price, then naturally you can expect them to do that too. However, planned obsolescence can often be used to squeeze more profits out of the equation. Especially if you have a monopoly on the product or if the customers are unable to tell that your product is degraded.
As far as costs go, the degraded product might be more expensive, but if it can be sold for enough profit, it can be worthwhile for the company to do so. However, planned obsolescence doesn’t always increase costs. Sometimes it decreases them as well. For example, if you can achieve the degraded product by using less or lower quality materials. Of course, this might benefit the customer too, if the savings show up in the price. However, it’s often the case that companies push this further than any customer would actually want if they understood what’s going on.