I think these numbers are very location specific. Those house flipper tv shows loved to toss them out because they make great graphics, but they are nonetheless subjective.
The only way I know how to guess the value of your home is to get comps. (What similar homes in your local region sold for recently, withing a small radius such as a quarter or half mile.) So to decide your ROI figure out your homes current resale value. Use one of dozens of home price estimators for this.
Example 100K house, estimated value after adding a bath $120K, but the bathroom cost $10K to install.
(120 / 100) - 1 = .2 or 20% increase in the home value. Increase is $20K, but since the bathroom cost $10K your ROI is only $10k. (is my math correct?)
But that doesn't tell you what new drapes, or updated carpeting will get you. Since you can't measure those numbers in any realistic way, you are stuck relying on the gut of a real estate agent, which isn't mathematical and therefore something I can't trust. For the little things, they are items that you would want to just make look and feel nice if you are selling the home.
Things I have learned:
- Avoid Junky Lowes or Home Depot fixtures and parts. If you buy the cheap ones then your home looks like every cruddy apartment building in your city.
- Hire a pro for things that need to pass inspection
- Get your permits. Upgrades don't count if they aren't documented
- Consider a big picture. I did one room at a time in my house and now the house reflects that. I should have planned the whole house stylewise, even if I only did one thing at a time.
- Never never never big the best or nicest house on the street. Don't be the worst, don't be the best.