Welcome to the forum
Out of curiosity - when you refer to the Samsung Galaxy - are you referring to the Galaxy S2 or the Galaxy S3. If it is the S3 you are referring to - the phone is currently only available to pre-order and because of that the prices you have been quoted aren't necessarily upgrade prices.
I am pretty sure one of the e-Forum team can send you a PM and arrange to contact you once the S3 is in stock to discuss the upgrade options available to you.
Hi trueblue924, Welcome to the eForum 
From the price plan you've mentioned it looks like you're interested in the Galaxy S II initially.
Third party websites can offer cheaper deals to connect you to the networks as they buy ther handsets themselves and so negotiate their own cost for the device - the commision they receive from the network for introducing you covers any potential loss 
The downside to this is if anything goes wrong with your handset you need to speak to them to get it fixed, rather than dealing with the network direct
Your upgrade quote is unique to you, and based on you as a customer with over 300 different variables taken into account when calculating your quote - did you get the £300 quote by going through the upgrade section of MyAccount?
I'd like to see what we can offer you for your upgrade so I've sent you a PM - follow the instructions on contacting us and a member of the team will be in touch as soon as they can for a chat about your options
You can find your PM inbox here
Paul
