Its not more expensive to secure a co-op loan, you just have limited options. For example Wells Fargo is one of the few lenders that are still providing loans for co-op purchases, but the cost and interest rate is the same as if purchasing a similarly priced condo. The only difference may come in the closing costs, as the maintenance and taxes of one unit, may differ to that of the other. Also, some buildings request 3 months of working capital up front, others 2 months etc.
In terms of re-selling a co-op, I believe that it is harder than selling a condo, and its mostly because people dont want to go through the board-approval process, and many times cant even qualify because they dont meet the 3:1 or 4:1 ratios that some boards require. This however is where price comes to play, and as you probably know, co-ops are usually priced lower than condos. Primarily because they dont really gain as much value compared to a condo, but also because of the high maintenance fees, as well as the income to debt ratio that a buyer needs to qualify. If its priced correctly, it will sell though. I just sold a co-op on Blvd East for $130k. 800sq.ft, updated, with a balcony, in a full service building. A similar condo would cost $70k+ more. So as long as its priced right, it will sell.
Hope I was able to answer your question.
Keller Williams Realty