How much do I need to spend to get a preamp that sounds better than no preamp?

All preamplifiers have a sonic signature.  Even passive devices have components that in some say affect or color the sound.  Perhaps a better question to ask is which preamplifier will compliment your systems shortcomings?  This may sound strange but if I'm right that all preamplifiers have a sonic signature, some obviously more than others, then running your components, if they posses volume control is adding the least number of components to say nothing of additional cables and connectors.  I use a switching device between my line level components.  This eliminates additional active or passive components but introduces additional cable length and a switch.  I've read here that some preamplifiers enhance the sound.  This is possible for many reasons and not just the source components' shortcomings. Preamplifiers where originally designed to buffer and add additional gain but today this is rarely the need as most source components with the exception of a turntable, need additional gain for buffering. If you're not running long cable lengths then no additional components will get you closer to the source components sonic signature (this assumes that you like that signature) and in the end a preamp regardless of type will have some impact on your system.