If you want to build a metal R2 then it wont be cheap. Many builders will easily spend over $10,000 on metal parts and electronics, spread over many years. You can find help with getting metal parts and components on the astromech site. But you will need to hunt around for someone who can help. The cheapest I found a metal dome in stock for was £600 plus postage to the UK from the US. And that was just for an uncut aluminum one. That said I have seen them for around £380 mark + postage, but they are rarely in stock at those prices (Granite Earth) Metal ones are the best looking and will last the longest but their are other options, that are cheaper, though still require a lot of work..