The cost of the Australian skills assessment is dictated by TRA (Trades Recognition Australia). You pay TRA directly and they have chosen RTOs (Registered Training Organisations) that deliver the Diesel Motor Mechanic skills assessment for Australian migration. The cost for the Documentary Assessment is $1280 and for the Technical Interview the cost is $2000.
UK: +44 (0)20 3376 1555
https://www.google.co.uk/search?q=Down+Under+Centre,+48+Haven+Green,+Ealing,+London+W5+2NX&ludocid=18176234724123169362#lrd=0x4876120a70f88a5d:0xfc3ef52e057b5252,3