Unless there are white rings around the tritium vials, you're not going to see much in the light. They are, after all, night sights. However, there are plenty of night sights on the market with rings around the vials that are more visible in the light. Some people prefer the vials to be near invisible in light, leaving a (primarily) black sight, and some like the presence of white or other colored rings/backgrounds. And there are a variety of configurations, too. If I use night sights, I prefer a tritium dot in the front sight only, with plain black rear sights. It's all personal preference.
I can't help with the cost of a M&P with night sights, but sights can be fairly easily replaced. Prices can range anywhere from $80 to $150, depending on which ones you get. While some people install the sights themselves, others, with less experience or skill, can have gunsmiths install them. In my experience, the cost is typically $50. However, some gunsmiths/gun stores will install them for free if you buy the sights from them.
Night sights can dim over time. Typically, they have a half-life of about 12 years for green dots, meaning that in 12 years time they'll be about half as bright as they were when they were manufactured (not when they were sold...gotta watch out for that). Some dots are colored yellow or orange. Those will have a little less life in them. I want to say a half-life of 10 years, but I could be wrong. I had a set of night sights in my Beretta 92FS, green front with yellow rear, and they're completely dead after 20+ years. For a M&P, you can just replace them when they get too dim for you.
Hope that helps.