I just published a PostgreSQL port of the Open Source Shakespeare database: https://github.com/catherinedevlin/opensourceshakespeare. It's much more fun than all those other sample databases.

The problem is everytime you run your command PostgreSQL took a seed number and send it to srand function. That is why with the same seed you get same values from your arrays. Therefore, you need call function as many times you want another value from your arrays, and you can do that creating functions. That will cause context switch, but that is the only thing that I can think of. Maybe someone else can correct me. Here is the code:

create table test (
    id serial, 
    contract varchar, 
    amount0 int, 
    amount1 int, 
    price double precision
);

CREATE OR REPLACE FUNCTION random_amount()
  RETURNS int
  LANGUAGE sql VOLATILE PARALLEL SAFE AS
$func$
  SELECT ('[0:4]={50, 60, 80, 100, 200}'::int[])[trunc(random()  * (4 - 1 + 1) + 1)::int];
$func$;

CREATE OR REPLACE FUNCTION random_price()
  RETURNS int
  LANGUAGE sql VOLATILE PARALLEL SAFE AS
$func$
  (SELECT ('[0:3]={1.5, 1.8, 2.1, 2.5}'::float[])[trunc(random()  * (3 - 1 + 1) + 1)::int]);
$func$;

INSERT INTO test (amount0, amount1, price)
SELECT
  random_amount(), random_amount(), random_price()
FROM generate_series(1, 100);

select * from test;

Fiddle.

Another method that can be used even when all columns have default expressions is to provide one column and use the OVERRIDING USER VALUE option in the INSERT in order to ignore those provided values.

insert into foo (id)
  OVERRIDING USER VALUE
select null
from generate_series(1, 10) as gs(i) ;

Tested in dbfiddle.uk (1)

One more method that uses MERGE - available only in Postgres version 15 (to be released next month). Thank you to Paul White who located the similar questions for SQL Server and Martin Smith and AndriyM for the answers:

• how do I insert a default row?
• Insert multiple rows into a table with only an IDENTITY column

Rewritten here with the necessary changes for Postgres and tested in dbfiddle.uk (2):

MERGE INTO
  foo AS tgt
USING
  generate_series(1, 10) AS src  -- << use your source row set here
ON
  FALSE
WHEN NOT MATCHED THEN
  INSERT DEFAULT VALUES
;

(Wish note updated)
It would be nice if we could provide a 0-column table as input but this has not been implemented yet - the select works on its own but not the insert into foo ():

insert into foo ()
select
from generate_series(1, 10) as gs(i) ;

But this works since version 9.4. The trick is to use insert into foo - without the () - which accepts an input table with arbitrary number of columns (from zero up to the table's columns number):

insert into foo
select
from generate_series(1, 10) as gs(i) ;

Testing all three methods in dbfiddle.uk (3):

select version();

SELECT 1

CREATE TABLE foo (
    id INT PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
    name TEXT DEFAULT md5(random()::text)
);

CREATE TABLE

insert into foo (id)
  OVERRIDING USER VALUE
select null
from generate_series(1, 3) as gs(i) ;   -- add 3 rows

select * from foo ;

INSERT 0 3

SELECT 3

MERGE INTO
  foo AS tgt
USING
  generate_series(1, 5) AS src   -- add 5 more rows
ON
  FALSE
WHEN NOT MATCHED THEN
  INSERT DEFAULT VALUES ;

select * from foo ;

MERGE 5

SELECT 8

Alternatively

MERGE INTO
  foo AS tgt
USING
  generate_series(1, 5) AS src   -- add 5 more rows
ON
  FALSE
WHEN NOT MATCHED THEN   
  INSERT (id) VALUES (DEFAULT);

insert into foo
select
from generate_series(1, 2) as gs(i) ;

select * from foo ;

INSERT 0 2

SELECT 10

fiddle