Looking purely at the value of the SUB, with the common redemption methods of booking through Chase Travel or booking an award fare (outside of sales) with Delta, even after paying the AF on CSP the first year you'd still come out $65 ahead, and the value of the points earned on CSP from spending would be worth a little more that way as well. They really aren't cards one would typically compare, as they offer substantially different benefits and earnings.
The Delta Gold will really only make sense past the first year if you normally pay to check 4 or more bags per year, or spend at least $10,000 a year on it to get the $100 flight credit that offsets the annual fee. Do you fly with a companion at least once a year? If so, perhaps consider the Platinum SkyMiles card instead. The Main Cabin companion certificate issued at each renewal should more than offset the annual fee, and the SUB is currently 90k on it. If you fly at least once a year with a companion in First, or if you have/want elite status and can spend $30k or more on the card each year, consider the Delta Reserve. The AF on that one would be completely taken care of with the companion certificate, it has an 80k SkyMiles + 20k MQM welcome offer currently, plus it would make you eligible for upgrades after Silver Medallions even if you don't have status, or be a tie-breaker if you do and another elite at the same level in the same fare class does not have the card.
20k SkyMiles would have a minimum award value of $240 for Main Cabin. The companion certificate doesn't really require loyalty, just one flight a year with a companion.
To get to the Delta Platinum Companion Certificate, yes you would pay $500 in the 13 months ( two AF ) to get there. But you'd pay $200 of that on a Delta Gold AF for two years, so the marginal fee to get to the first Companion Cert is $300. However that Companion Cert is only Main Cabin, whereas Delta Reserve has First Class.
I went through the Delta cards list over several years, used the Companion Certs on both DP and DR. The sign up miles are useful if Delta is your main airline, so I'd suggest approaching it as a series of try-outs rather than trying to get the one Delta card you want forever.
