How do you know if a vendor's price is actually competitive?

Get multiple quotes for comparison
The only reliable way to know if a price is competitive is to gather 3-5 quotes for the same or similar work. This gives you a range. Prices clustered together indicate market rate; one outlier might be a red flag—either underpriced or overpriced.
Understand what's included
Two vendors quoting different prices might be offering different scopes. One might include materials and warranty; another might not. Break down each quote line-by-line to compare identical services. A 20% price difference disappears when you account for what's actually included.
Check market rates informally
Talk to people who've hired similar vendors recently. Ask friends what they paid. Search online forums where people discuss typical costs. This informal research helps you develop intuition about whether you're in the ballpark.
Account for vendor reputation and speed
A more experienced vendor might charge more because their work is better or faster. Lower price doesn't mean better value if the cheaper vendor takes twice as long or does mediocre work. The cheapest option is rarely the best deal.
Red flags in pricing
A quote significantly lower than others might indicate the vendor under-estimates projects or cuts corners. A quote significantly higher than others might be inflated. Both warrant questions: "Why is your price different from others I've received?"