Difference from average

Hi all,

I'm developing a property rental database. One of the tables tracks
the price per week for different properties:

CREATE TABLE "public"."prices" ( "id" SERIAL, "property_id" INTEGER, "start_date" TIMESTAMP WITHOUT TIME ZONE,
"end_date"TIMESTAMP WITHOUT TIME ZONE, "price" DOUBLE PRECISION NOT NULL 
) WITH OIDS;

CREATE INDEX "prices_idx" ON "public"."prices" USING btree ("property_id");

I'd like to display the prices per property in a table, with each row
coloured different shades; darker shades representing the more
expensive periods for that property. To do this, I propose to
calculate the percentage difference of each rows price from the
average for that property, so if for example I have two rows, one for
price=200 and one for price=300, i'd like to retrieve both records
along with the calculated field indicating that the rows are -20%,
+20% from the average, respectively.

I've started with the following query, but since I'm still learning
how PostgreSQL works, I'm confused as to the efficiency of the
following statement:

SELECT *, (price - (SELECT avg(price) from prices)) as diff FROM prices;

EXPLAIN reveals (albeit not a real test, as only the two rows above)

Seq Scan on prices  (cost=1.03..2.05 rows=2 width=32) InitPlan   ->  Aggregate  (cost=1.03..1.03 rows=1 width=8)
->  Seq Scan on prices  (cost=0.00..1.02 rows=2 width=8) 

Does this mean that I'll be performing a nested table scan every time
I run this query? Also, I haven't yet calculated the percentage
difference for this, which in my eyes means another instance of
"SELECT avg(price) from prices". Is this the best way of doing this?
Can I optimize this away by re-writing this as a function and storing
"SELECT avg(price) from prices)" in a variable?

All opinions gratefully received.

Kind Regards,

Neil