$filter with lookup ids; SDK.REST call failing

I've got a few situations where my select string for a retrieveMultipleRecords call contains filters that include lookups (entity references).  An example is below which contains 3 lookups in one filter.  For example, I have the GUID for a Price Level, and I want in my filter for the field PriceLevelId to equal that GUID.  But you cannot set PriceLevelId = theGUID because PriceLevelId is an entity reference (or object).  But you can theoretically get that object's Id with PriceLevelId.Id.  But neither works.  What am I missing?  Is there a way to accomplish the goal of this code below?  (For clarity, the error is in the selectString declaration).

function getPrice2() { //currently not working
    var pricelevelid = Xrm.Page.getAttribute("pricelistid").getValue()[0].id;
    var uomid = Xrm.Page.getAttribute("unitid").getValue()[0].id;
    var productid = Xrm.Page.getAttribute("productid").getValue()[0].id;
    var priceInfo = null;
    if ((pricelevelid != null) && (uomid != null) && (productid != null)) {
        var selectString = "$select=Amount,ProductPriceLevelId&$filter=(PriceLevelId.Id eq guid'" + pricelevelid + "') and (UoMId eq guid.Id'" + uomid + "') and (ProductId.Id eq guid'" + productid + "')";
        SDK.REST.retrieveMultipleRecords
        (
            "ProductPriceLevel",
            selectString,
            function (results) {
                if (results != null && results[0] != null) {
                    priceInfo = results[0];
                }
            },
            function (error) {
                alert("getPrice Error: " + error.message);
            },
            function () { },
            false
        );
    }
    if (priceInfo) {
        return priceInfo.Amount;
    }
    else {
        return 0;
    }
}

Mark Rockwell