Assuming that a commodity fulfills your needs, here are some precise questions to help answering the question if it is too expensive.
“Could I easily lend this item?”
“Could I leave this item unattended?”
“Could I ship this item without stress?”
“Would I fret if this item was broken?”
“Could I re-purchase this item if it was stolen?”
If you mostly confirmed, it probably means that the item in question is within your budget. Otherwise, it is probably too expensive for your current financial situation.
Just because an item is within your budget, it doesn’t mean that you need it though – check my approach to Possessional Minimalism . If you feel like you are seeking satisfaction via luxury, you might want to read Two Factor Theory: An Approach to Life Satisfaction .
If you are a musician, I recommend reading the related article Do You Need An Expensive Guitar? .