At one stage here we were thinking of selling our house and got the house valued. We spoke about doing up the kitchen believing that it would add value to the house. The valuer told us that it was usually better to leave the kitchen as is and keep the cost of the house down. I guess that you may lose some potential buyers by not having a nice modern kitchen, but you may also spend the money on the new kitchen and lose some potential buyers because the house is either outside their price range, or they don't like the design and layout of your new kitchen. Sometimes, if you have a real knack at interior design and do a really good job then new kitchen can add more value to the house than the kitchen cost.
When I add farkles to my bike (or 4WD) I try to avoid making any permanent modifications to the basic vehicle even to the point of avoiding drilling new holes (can I do it using existing screws, bolts, brackets, etc?). I do this in case I decide that I don't like the new mod and/or so that mods can be removed without leaving scars. I have found with 4WDs that often you get more of your money back by removing the mods and selling car and mod separately. Like the example above.. sometimes some mods that are well thought out and well installed and presented can/will add more value than it cost. I have seen mods on bike and 4WD forums where the owner/installer believed that it was a beaut mod, but looked really badly done to me.. not saying that is the case here (because most look really good to me), but sometimes beauty is in the eye of the beerholder!

From my own point of view: When I bought my ST it was from the original owner and had low k's. I was disappointed when I got it home to find bare sections of cables either side of the steering head and that they were lucky to have some small amount of electrical tape protecting them. I hate that. Has water made it's way inside the insulation and started corrosion that may cause an intermittent or permanent fault down the track? So, for me I would prefer my vehicles to be unmodified unless the mod or farkle really belongs and is well installed and presented.
To be quite honest, I have looked at all the mods/farkles that are available for the ST1100 and have taken an interest and given each one good thought. AFter having my bike for 3 years (?) I still have few noticeable farkles until I start to think about it. So far, most of the farkles I haven't needed, and I have less worry about bits coming loose, being lost, stolen, or weather damaged.
Most of my farkles are things that I felt the bike needed for me: Quick connect for my multi-stage battery charger, handlebar risers, wind deflectors (mirror and side fairing), 12v outlet to handlebars, custom seat. I have kept the handlebar cover and bits in case it ever needs to be reverted back and the risers removed.