I agree with Viper, there is so much hype on the price/size of a particular coral...There are a couple of sites that sell maricultured corals which are billed as "this or that" the colors are not as intense and they typically do not look like the "originals" I've learned my lesson in this area, it's better to go for quality vs size IMO.....
Now, if the coral your talking about is one of those screaming deals you stumble across everyonce in while and the colors are true to what a good quality Rainbow should look like, then it's an incredible deal. If it's just a monti that has colors that are close to a rainbow, then it's totally based on what YOU think it's worth. IMO most of the rainbows that are that size don't look that great after a period of time, the frags look awesome, but once the new growth has been around for a while they tend to lose their coloring.