That would depend on how old they are and what condition they are in. I have bought used washers and dryers in the past and spent about $125 each for them, but they were in excellent condition. I have seen them in papers for $75 on up. I would consider the price of a new on when deciding what to sell yours for. Lowes has basic new dryers for $300 and basic new washers for $375
If your washer and dryer are in good condition, have not been beaten up or overly used, you can get close to half the cost that you paid. If they are in like new condition, I would set the price at 75% of the original price.