As others have hinted at its all very well guestimating the likely length of fence but unless u go out and put 'eye's' on it u could be well out, if loads of small wee kinks so loads of strainers, acces, water gates etc loads of stuff that will all eat into ur profit, ur talking 30K but if u get it wrong and jobs takes bit longer (or aa lot longer) extra materials etc could end up costing u a fair bit
Be well worth walking the 3km prob take an hour, esp so if ur new to deer fencing or don't know the area
Not being cheeky but plenty of people get their fingers burnt doing upland fencing, never mind deer fencing, have to know wot ur doing and have the gear for getting materials out there