Issued debt typically trades near par (100) unless the borrower's credit rating has changed or interest rates have changed (both of which happen all the time, but still, the vast majority of debt trades in the 90-110 range.)
You can compare two companies with different levels of leverage. They both have Enterprise Value (EV), which is normally calculated as operating income (EBITDA to be precise) * a multiplier. Market capitalization, or equity value, is by definition the different between the EV and the net debt (debt - cash). (P/E ratios are usually not as useful as the EV/EBITDA approach for understanding a company, btw.)
One company may be highly debt financed and the other may be pure equity financed. As you say, debt financing supercharges equity earnings (see below), but this comes at a higher risk. And in principle, the higher earnings are balanced out by the higher risk and therefore enterprise value is unchanged.
What is a bit harder is comparing two companies in entirely different industries, like Ford and FB!
Equity returns are supercharged by debt because in a growing company debt coupons are cheaper than equity returns. e.g., say you invest $100 to build a company that makes $20 a year. You get a 20% equity return. But imagine you invest $50 and borrow $50 to build a company that makes $20 a year. Your debt pays a typical 5% coupon, so you pay 50*5% = $2.5 in interest. The remaining $17.5 goes to your equity and you get a 35% equity return. BUT if you're unlucky and your earnings are delayed one quarter, it's the bank that gets your 35% equity return cuz you bankrupt, sucka.