I believe the XP earned is exactly 1 XP per $10 spent, regardless of what item you buy. You will get your multiplier (star rating + premium items) added and then it rounds up to the nearest whole number.
So depending on what your multiplier is, the exact best value will vary somewhat. You want cheaper items to maximize the number of times you get to round up (think "Office Space"), but need items that put you just over a whole number so that your rounding up bonus is maximized.
Knowing your EXACT multiplier is helpful here, which can be discovered by collecting from very high-paying buildings or long tasks.
The more I've done this as my multiplier has increased, the more variable the ROI seems to be, so I'm planning to recheck my numbers once I reach 50%. I might also redo things with hiding certain items and getting different multiplier levels to see how the rounding up or down affects the ratio between items, but that's pretty time-consuming. We'll see how that goes.
I put together a tool that I used to figure out what the best conversion is for me. I uploaded it to Google Docs so that anyone who wants can use it too.
The first tab will help you to figure out your exact multiplier. Enter a few cash payouts (stated and actual w/ bonus) to narrow down your exact multiplier. Cash works better than XP because the multiplier is the same, but the effect of rounding is less noticeable on the larger sums of cash.
You can enter this multiplier in the second tab to calculate which items will give you the most XP for your cash, maximizing your benefit from rounding.
Strange... I got a neighbor with 3 buildings in a row which I can vandalize. I've tried multiple times, never works for me...
Are you tapping another building first? Sprays need to be 2-4.
Are you tapping fast enough? Must tap 2&3 while 1 is still spraying.
Other than that, I don't know why it wouldn't work.
I haven't been able to find 3+ buildings to vandalize close enough together since the Valentines update, but I do recall that when the animation finishes on the 2nd tap (or if you tap two buildings before vandalizing), the instant you tap a 3rd time the rest of the icons disappear.
You have only the length of the animation (spraying/cleaning) for your second tap to complete taps 3, 4, and 5.
5 taps requires all four vandalizable buildings to be on one screen, while 4 taps requires that at least three of them be within the screen.