If you're price sensitive, just monitor performancebike.com, nashbar.com and amazon.com for a few weeks and watch for a sale. If you're not too rack savvy at the moment, I'd recommend choosing from racks that have decent reviews. The thing is, at the lower price point, designs and brands come and go, so at the bottom price range you're not going to see an iconic brand like Ortleib or Blackburn, where you can be assured the rack is good.
But using the above method, I've consistently been able to purchase decent rear racks in the sub-$25 range, over the past 4 years or so. Front racks are much less common to find in this price range, at least when I've been looking, but I did snag one on sale from Nashbar a couple of years ago that was <$20. On one of my touring bikes, I managed to adapt an inexpensive rear rack to the front of the bike, taking advantage of the extra eyelets on the fork, and the cantilever brakes.
You're definitely on the right track trying to distribute the load between front and rear. This helps with wear and tear on the rear wheel, and to some extent with balance and handling. A large handlebar bag can help with load distribution as well, I would suggest in addition to a front rack & panniers, rather than instead of.
To be clear, the front racks I'm familiar with and have been alluding to are for panniers and perhaps a small auxiliary compartment on top, not for a large porteur style setup. The porteur setup is considered to have a desirable element of style, but, while I don't know, I suspect that a decent one (i.e. rack + compatible basket or bag) is not cheap to purchase new from a retail outlet. And with the amount of weight the porteur rig is potentially capable of carrying, it's not a given that any old bike will handle well with it loaded to its max. If there isn't theory or anecdotal information that suggests your particular mixte will handle well with a full porteur load, I wouldn't necessarily make the leap of faith that it will.