trigger does not work for some values

create table proj_ect(projno int not null,projname varchar(20) not null,budget numeric(10,3) not null)
insert into proj_ect values(3,'gdf',30000.000)

create trigger ducati_monster
on proj_ect
for update
as
begin
begin transaction

if((select budget from inserted)>(select sum(budget)*.5 from proj_ect))
begin
rollback transaction
print'value is above the permitted range'
end
else
begin
print'budget updated'
commit transaction
end
end

update proj_ect set budget=61 where projname='prithvi'


here the trigger does not work properly for some values.can anyone help to find why it is so

thanks in advance