$600 for an A10/A65 bare frame is a lot of money in my eyes but I suppose they're getting a bit scarse. It'd make a great starter for a TRIBSA special as Marc says. At one stage prior to the advent of purpose built frames the TRIBSA was the hot ticket for scrambles stardom.
I reckon the basic TRIBSA shown could be built for around 5k using a bit of savvy eBay and old mates network shopping.
Assuming you get the frame for $600 and using eBay USA pricing for parts here's how I break it down
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Frame: Top dollar at $600
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forks/triple clamps/front wheel: Bultaco Betor forks and full width hub @ wheel $200
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rear wheel/shocks: I'd use a Rickman Montesa rear wheel $100. Period sympathetic Konis or Girlings can be had for under $50 or see Walter for some decent YSS UNITS. keeping it cheap though, allow $100 for second hand whatever.
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Engine:rather than using a pre unit engine and AMC or BSA box like most, I'd use a cheaper and simpler and mor common T100 unit 500 I've seen complete engines for as little as $400us but allow $1000au.
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tank/seat: Replica Metisse bodywork can be bought for under $500 or for a real cheapo you could use an alloy Husky tank and seat for around $200.
That brings the basic bike in at around 2.5k. Double that for engine freshen up, paint, polish and upholstery, tyres and possible wheel rebuild and you've got a very basic TRIBSA up and on the track for around 5k. Many will do it cheaper, some will spend a lot more but on average you
should be able to get a basic low bling TRIBSA on the track. Building a bike like a TRIBSA allows you to use a bit of artistic flair so why be restricted by someone elses idea of what's needed. Why not use a Yamaha MX125 front wheel (
thanks for the tip Graeme) which already comes with a mudcatcher rim? Why not an alloy RM Suzuki tank polished up? What about CZ wheels and forks?
As you can see, the component list to build a TRIBSA is almost endless using the above list of eligible bikes and their parts. Below is a shot of Kevlar Herbies TRIBSA bling bike that has been fitted with Metisse bodywork to replicate a Metisse Mk2........like I said, the scope for parts choices is almost endless.