long interconnect cable suggestions?

You did not state whether you need single ended or balanced for this run. If you are using true balanced, the impedance would be 600 ohms, and technically may be run almost any length, from any good manufacturer. If you are using a single ended (RCA connection), you need interconnect that does not have excessive capacitance. Otherwise, you will suppress the signal bandwidth of this long, 12 meter run, due to excessive loading to your preamp. One of the lowest capacitance cables is my favorite, the Purist Audio Design. The entry level Elementa, (at 17 Pf per foot!) may be ordered to your exact required length, and at retail (less 10 %), you should pay about $780.00. I realize this is more than you would like to spend, but finding a quality 40 foot interconnect used will be very difficult. In fact, Inexpensive, high performance, low capacitance, interconnect cables are limited, even as a new purchase. I am using a 40 foot run of Purist in my own system, only after having experimented with a number of other name brand interconnects. The Purist remains in my system for the very reasons I am explaining. I might add, that I do prefer this "system" of long interconnects and short speaker cables. From other posts I have read here at Audiogon, many other posters have reached this same conclusion. Best wishes!

Take a look at Canare for single-ended. I have a 30' run and a neighbor who is a ham radio operator. Zero interference or noise that I can trace to the cables. BTW, the cable is between a McIntosh C-38 pre and a McCormack DNA 1/B. Enjoy!

I second Dave's answer. Canare makes great, studio-grade cable, and it would be well within your budget. If you're driving the line with a low-impedance unbalanced source, you're likely to have less signal degradation than from a passive, high-impedance source. For relatively long runs like yours, though, balanced interconnects are several orders of magnitude cleaner and more trouble-free than unbalanced. Whenever possible, go balanced.