How do you mean by "How much the swap is worth"?, And honestly I have recently gone from an RCD300 to an RNS510 and it was so simple and easy to do that paying an auto electrician seems criminal! There is guides online on how to remove the trim which is the hardest bit, (Invest in a £5 trim removal kit) and make sure you buy the aerial adapter as the RCD300 and the RNS510 aerial ports are different (The adapter is a plug n play swap - easy!) and the GPS unit is really simple also, I have mine situated just behind my speedo dials and I get perfect reception everywhere!