I do not know how to solve this problem without scripts, but there is a simple work around. When you update prices, you can drag them elsewhere in your sheet (e.g. below, in the form of a price record). The values on the 'Sales' sheet will stay linked to the original price, and the formula you enter for sales will be the same.

If you choose this work around, I would recommend creating a script that automatically moves your prices so that you don't have to manually drag things around and so that the newest prices appear closer to the top. There are many ways to make that type of script, but here's one not so elegant, but simple way:

var ss = SpreadsheetApp.getActiver();

// Many of these values depend on the layout of your document.
// I have indicated where you will need to change values if
// you change the layout.

function onOpen(){
  // This will make a menu appear that allows you to run the scripts
  var ss = SpreadsheetApp.getActive();
  var items = [
    {name: 'Make space for a new price', functionName: 'newprice'},
    {name: 'New sales day', functionName: 'newsales'},
  ];
  ss.addMenu('New Entry', items);
};

function newprice(){
  // This will simply insert a row under your prices.
  // You just have to drag the on you want to change into the new row
  // Your info on the other sheets will stay linked to the correct number
  var sheet = ss.getSheets()[1]; // This number may need editing
  sheet.insertRowBefore(7); // This number may also need attention
};

function newsales(){
  var sheet = ss.getSheets()[0];
  var where = 7;  // Where to add the new lines?
  var how_many = 5;  // How many products to add?
  for (var ind = 0; ind < how_many; ind ++){
    sheet.insertRowBefore(where);
  };
  // Copy the values
  var source = sheet.getRange("A3:G7"); // This would need attention
  var destination = sheet.getRange("A8:G12");  // And this...
  source.copyValuesToRange(destination,1,1,1,1);
};

Using a script may be more trouble than it's worth for this type of problem.

If for some reason (I can't imagine why) you want to have one cell that adds old prices and new prices, then you need a more advanced script. It would be easier to make one cell associated with each price, and then add them in the final cell.

You can use the following script (see Tools > Script Editor). It executes whenever the spreadsheet is opened, and searches column A for the current date. It then sets the cursor to the cell with that date. You can use another column instead of A by editing the command sheet.getRange("A:A")

function onOpen() {
 var ss = SpreadsheetApp.getActiveSpreadsheet();
 var sheet = ss.getActiveSheet();
 var range = sheet.getRange("A:A");
 var values = range.getValues();  
 var day = 24*3600*1000;  
 var today = parseInt((new Date().setHours(0,0,0,0))/day);  
 var ssdate; 
 for (var i=0; i<values.length; i++) {
   try {
     ssdate = values[i][0].getTime()/day;
   }
   catch(e) {
   }
   if (ssdate && Math.floor(ssdate) == today) {
     sheet.setActiveRange(range.offset(i,0,1,1));
     break;
   }    
 }
}

This script borrows from this answer by Serge insas.

Unfortunately, it seems impossible to bind a script to the event of switching between sheets. So, if you'd like to invoke it on more than one sheet, you'll need to do this manually. The following version, in addition to doing what's described above, also adds a new menu item Custom > Jump to today's date, which you can use on any sheet. This also allows you to return to the date if you wandered off in the process of working with the sheet.

function onOpen() {
 var menu = [{name: "Jump to today's date", functionName: "jumpToDate"}];
 SpreadsheetApp.getActiveSpreadsheet().addMenu("Custom", menu);
 jumpToDate();
}

function jumpToDate() {
 var ss = SpreadsheetApp.getActiveSpreadsheet();
 var sheet = ss.getActiveSheet();
 var range = sheet.getRange("A:A");
 var values = range.getValues();  
 var day = 24*3600*1000;  
 var today = parseInt((new Date().setHours(0,0,0,0))/day);  
 var ssdate; 
 for (var i=0; i<values.length; i++) {
   try {
     ssdate = values[i][0].getTime()/day;
   }
   catch(e) {
   }
   if (ssdate && Math.floor(ssdate) == today) {
     sheet.setActiveRange(range.offset(i,0,1,1));
     break;
   }    
 }
}