I don't spam trade or tell anyone that I sell portals. I get tons of whispers though, because I don't raid and I play the Auction House a lot so I'm almost always in stormwind.
My rates
20g per person - You come to me
40g per person - If I have to port to you (ie dalaran to stormwind)
I get a complaint from maybe 1 in every 10 people, but they always end up paying. I've had tips of upto 50g-100g for a portal. One person even paid 500g once.
Do you think my rates are worth it? Please think not just of the cost of the rune, but think of it this way. I am going out of my way to stop everything I am doing to make your life easier...that is a class benefit of a mage that we should always have because we're the masters of instant-travel. It's a shame one of our main class selling points was made useless in WOTLK. At least now we're back to what we once were. Thanks blizzard for not putting portals everywhere like you did in WOTLK .