I spent yesterday in Malibu, doing a little renovation consult for a friend who recently purchased a trailer home right on the ocean (more on the trailer home thing in a bit; it's a super cool story). It's an interesting undertaking, because the house is going to be torn down and rebuilt from the ground up in a couple of years, so the renovation is more about making the house pretty and livable for the short term, without spending tons of money on things that'll ultimately end up getting tossed.
So. My advice was to figure out what amount of money per month was a comfortable amount to spend on general Life Happiness (meaning an amount that creates no financial stress whatsoever, but that feels "worth it" for the lifestyle improvements it'll confer), and multiply that by 24 to arrive at a budget for the mini-reno that'll take him through the next two years. Certain things are OK to spend a little more on, IMO - appliances that'll make the jump to the new place, and so forth. But when it comes to structural features, things are a little trickier.