A rough guide is the nicehash one....At the moment with the price crash it's all gone a bit scary. However most believe that the profits will rise again.
Even mining when not officially profitable vs your electricity costs, you can be buying altcoins at a discount to buying on an exchange!
My S7's are probably not profitable at a day rate to electricty at the moment....My S9 is probably still good and vs the price of importing from Bitmain may still pay off it's costs. My 4xGT970 rig was only a grand as still would probably pay for itself in hardware breakdown, but mining things like Electronium and Trezarcoin which have huge growth potential could pay off and is easily covering it's electricity costs and boosting the heat in my house lowering my overall energy costs too.....It's all more of a gamble than a sure fire way to make money, but the odds look a lot better than getting ~1% in an isa....I've also a few things like nutmeg, property partner, rate setter and a fair comfort buffer in a cash isa that means I don't need to earn income from this and my lifestyle won't be effected if it all goes to zero, but it could dramatically improve things in the long game!
What happens when you can't comfortably buy an decent D3 S8 (or D4 A8)
(and you should really sell a few old cars before buying anything else)