If no one has specifically asked you for these letters - I can't see them being a big seller.
It seems quite tacky for a wedding... more in line with a Bat Mitzvah. I can get more like $1k for stuff like that but I take custom builds - actually doing the kids name, or some logo. They are a one-time use, and yes - foam core is the primary material. Sometimes custom lazer cut mirrors or plexi, MDF, etc if the prop has to be durable or part of some rigging.
You're better off building custom graphics (something not for sale to everyone) than trying to rent out the same cliche' word over and over. What people really want is for their event to be unique and if they have the budget to do that they won't want left over vocabulary for rent.
Some stuff like "Hollywood" is reusable and historic accuracy is fixed - so it makes sense to build it out of rugged material. But for personal events like weddings there is always some kind of personal twist, a different font, certain color, different spelling, etc. People will pay to have their own custom make. I see a variety of these stock "Love" letters in smaller versions at all of the home stores, and gift shops. I think there are plenty of ways for people to achieve a similar or more tasteful version of this word for under $95. IMHO bigger isn't necessarily going to be the attraction.