If reported costs are to be believed, the actual construction cost for FC big stand's big stand was around £75 million, plus 40 million for the extra land required. Seeing they were hampered by having to work around the existing stand instead of a fresh build that figure is probably higher than it needs to be despite the fact the lower tier was already in place. (It however was extensively remodelled)
So if you built a simple four sided stadium as per their main stand, (its capacity is over 20,000) knowing that the total land cost for the dock is believed to be in the region of £30 million, then it is possible £300M could build you something around the 70,000 capacity mark.
Obviously there will be extra costs for us as we will have the corners filled in and have to prepare the site, but the reduction of the capacity and the fact that materials can be shipped in directly should help neutralise that. So based on that a budget of 300 to 400 million doesn't seem crazy.