When placing spread orders to roll a position, never trade at the market's price when the bid/ask spread is that wide. The midpoint is a reasonable place to expect a fill and I would start start working the order at an even better price.
Suppose the quote for a diagonal roll of a long call (up and out) is $3.25 x $3.75. Selling the spread for $3.25 means that you would be selling the call at a lower strike and buying a call at a higher strike for a later expiration for a credit of $3.25 if you traded at the market. $3.50 would be a reasonable expectation for a fill.
It's not clear from your description what your quote of -8.3/0.5 represents. Is that the market quote for the spread order to roll your long call up and out? While quotes are usually positive (-8.3 ?), custom combo orders can be negative (and orders where the legs have very wide B/A spreads) so more details are necessary. If you want an accurate answer, you should provide the bid/ask quotes for the two options that you are looking at.
And no, you will not get more than the strike price difference when rolling up. Not even the strike price difference. And if rolling to a later expiration, it will be much less since you'll be paying more time premium for the longer dated call.