In season 4, Skyler was very adamant that they avoid large expenses that “contradicts” their story that they are broke to the IRS. She got upset when Walt bought the expensive wine bottle, but even more so when Walt bought the challenger for Walt Jr. The huge issue I have with that is at this point in the series they have already reported fake casino winnings to the IRS and then proceeded to buy a 800,000 dollar car wash. Relative to that, the challenger, and especially the the wine bottle, are cheap. It doesn’t contradict anything they are saying by Walt buying it, if anything it reinforces. Skyler is a bookkeeper, she would know this. It doesn’t make much sense.
I'm guessing Skyler's story goes like this:
Walt won a bunch of money gambling, which caused issues in their marriage. The Whites reinvested all of that into the car wash, medical bills, and Hank's rehabilitation. Walt is now in recovery and not gambling. The family's sole income is made from the car wash.
If this is true, it's more likely for Skyler and Walt to spend conservatively as they've done their entire lives instead of extravagantly.
They can hide in plain sight if it looks like they're making car wash money. As long as their lifestyle looks similar to what they had before, people will assume it's just the same old Walt and Skyler.
Questions like "When did you get into fancy cars?" and "When did you get into fancy wine?" mean they have to lie more, which means Walt becomes a greater liability bc he's such a terrible liar.