Well, I paid
$24.80 for circuit board (assembled board)
$ 8.00 shipping
$25.00 for r-core transformer
$14.00 shipping
$ 9.90 for aluminum feet
$ 7.00 shipping

Total $88.70
Plus the aluminum chassis, power switch, volume control knob, input plugs and the power plug with fuse I bought locally probably cost about $30. So the total cost should be around $120.

I was lucky to find a relatively good looking enclosure with just the right size (especially its height). The aluminum amplifier feet are not a necessity of course but it does look good and professional with them. I had to decide where to put the tiny power on led. Putting it above the on/off switch would have been much too common. I decided to gamble a bit with the design and modified the volume control knob, drilled it and inserted a blue led instead of the supplied red led on its back.

The links for the components I used:
Assembled circuit board
Assembled Test JHL Headphone Amplifier Class A Preamp DIY | eBay
R-core transformer
115V 230V 30W R Core Transformer 15V 15V for Headphon Amp Preamp | eBay
Aluminum amplifier feet
4 Pcs Aluminum Feets for Power Amp with Rubber Ring D 30mm H 12mm | eBay

Same circuit is also offered in a ready-made form in a chassis from various vendors.
But I enjoyed building it myself. It’s got a personalized touch.
BTW, I live in Izmir, Turkey. In my country personal imports are duty free up to €75 (~$100, including shipping I suppose). Buying it ready-made would go beyond that limit. I wouldn’t mind paying duty tax but bureaucracy to do that would kill me.

Building it shouldn’t take more than a few days. I mounted the circuit board on four pc mainboard spacers. Drilling a hole on the front panel at the right point for the potentiometer shaft was a bit critical. Also making a square hole for the power plug was a little time consuming. I drilled tiny holes along the lines of the square and filed the hole afterwards.

One important note: The heat-sinks on the board do really get hot. I plan to find a way to provide better air circulation inside the chassis. It may be a problem in hottest days of summer. When I first started the project I was startled when I saw a 30W transformer was recommended for a headphone amplifier. But then, this is a class A amplifier with relatively higher consumption and heat dissipation.