The costs of finding oil are on the rise. The value of some smaller oil companies has tumbled. For the world’s biggest crude producers, this adds up to a question: Is it cheaper to buy someone else’s oil than to go digging for it? As Exxon Mobil Corp. XOM 0.08 % and Chevron Corp. CVX 0.55 % report quarterly profits this week, executives are likely to face questions about their appetite for megadeals like the $70 billion takeover Royal Dutch Shell RDSA 0.61 % PLC disclosed earlier this month of BG Group BG. 0.56 % PLC. “There is no doubt that it’s much, much less expensive to take over a company than develop a new oil […]