Here are a few things to keep in mind. First, do not worry about "sounding unreasonable" when you're making an offer. The seller can either accept the offer, reject it, or make a counteroffer. And what one seller will consider unreasonable, another will consider reasonable. You're not a mind reader; I'm not a mind reader. I have no idea what a seller considers reasonable, nor do you. You should make an offer based on what you can afford, based on the value of the house to you (if you love it, you might offer more; if you're ambivalent about it, you'd probably offer less), while at least taking into consideration what the comps are. So, yes, if all the comps are coming in around $500,000, an offer of $250,000 would strike most sellers as unreasonable. But I'm not going to try and guess whether the seller would consider $500,000 unreasonable, or $475,000, or $450,000, or $425,000.
And if you do "sound unreasonable," so what?
There are statistics out there, by market, on sales price as a percentage of list price. They'll vary, but often will be in the 92%-95% range. But that doesn't mean a thing. It doesn't reflect the original list price. Someone starts out at $600,000...drops the price to $500,000 over a 6 month period, then sells for $475,000. Did it sell for 95% of list? Technically, yes. Practically, no. Or...does the selling price reflect seller subsidy? Depending on the loan program and permissible limits, sellers sometimes are "kicking in" 3% or more to the buyer. So the $500,000 house sells for $475,000...but there was a $12,000 seller subsidy. Now we have a house that started off at $600,000 that someone actually bought for $463,000. Furthermore, those "95%" figures are averages. Some houses may have sold for 100% of the asking price. Others sold for 90% or less.
I do agree with JR that you need to know what the comps are, what the good prices are. Realistically, if you offer 60% of what comps suggest the property is worth, you'll probably get pretty frustrated by the rejections. Still, I'd suggest using those figures as the ceiling price you offer. You don't want to overpay, especially in this market. So, know what the prices are, know what you can afford, determine how much you want the house, and then determine your offer.
Hope that helps.