How Much is This PC Worth ?

Solution
Your processor is very nice and GTX 970's cost has gone quite high these days. So sell it for 800€ if the parts are in good condition.
About the monitor, if you're not going to use it then you should sell it of course.
all depends who you sell it to if they know the value of it or not--sometimes on ebay for example things sell for more than they are worth because some buyers dont do their research

personally on ebay without monitor i would look to pay around £750 for it so roughly 60% to 65% of what it would be new