I want your opinion on masking domains.

I've got a fairly small site that I set up, and don't want to purchase a new hosting plan for it, so I'm just forwarding the domain to a folder within one of my other domains, and I'm masking it so it looks like you're at that domain. But this makes the pages load slow, and you can't refresh pages, cause when you do, it just takes you back to the main page of the site, and other things. So I took off the masking for now.

But what do you think about that? Is it extremely unprofessional to do that? For instance, when you point your browser to www.domain1.com, you get to the site, but in the address bar, it will say, www.domain2.com/domain1, or something to that effect.

Any thoughts? Or perhaps ideas for extremely cheap hosting? I mean, really all I need is maybe a couple hundred MB of space, if that, and the ability to run php and mysql databases. I currently host my other domains at www.globat.com, and I love them, but they don't really have any cheaper plans.