50 foot cables?

Hello,
With respect to your price range, I would highly recommend Belden 1313A speaker cable. This is a 10awg cable with oxygen-free copper and low capacitance and resistance, you should be able to go the whole 50'. I must admit though, that I am not familiar with Tube Amps. That said, for the distance you are trying to cover, this stuff should do the trick!

Good luck!

Rogue's manual gives no spec on output impedence, but most tube amps are higher than most SS amps in general, meaning less damping is applied to control the woofer of your speaker.  Longer cables add series resistance, which effectively reduces damping further, so minimizing that is an object of interest.  Belden 1313A is a good generic twisted pair 12 ga. wire, well under your stated budget.  You could go upscale and get Transparent HP122 for (just) within your budget, but only if you go under the flooring to halve the length. Its main difference is the extruded jacket that maintains the uniformity of twist of the conductors far better than the tubing used by most jacketed wire.  Later, as an upgrade, you can get a pair of their network modules ("Bricks").   
Drill, Baby, Drill!

I would use Canare 4S11 speaker cables from Ram Electronics.  A 25 foot pair with termination will run you $139.00 for the pair.  These blow away my $1500.00 pair of speaker cables. 

@OP, 
You might get lucky searching Ebay for used cables.
25 feet is long, but I scored a set of AQ Rockefeller cables a while ago.
They were the real deal and AQ cut them down and had them reterminated when I moved to monoblocks.
Also, check Hifi Shark. Use the filters to narrow down your search.
A little patience, and some luck should allow you to find a pair of cables.
(Just keep in mind, there are counterfeits out their, so do your due diligence. -That is try to find sellers who can provide a receipt of purchase from a dealer, if possible).
Bob