screach on start up

it's probably your alternator belt. it's worn to much or it's to loose. if loose that means your tensioner is not working properly. the screech is caused by the alternator going to full output after the drain placed on battery from starter. it only lasts a few seconds. you can try belt dressing as a temp fix if tension feels good. (belt is taught/tight)

is a tensioner expensive to replace ?

you could have someone start car while you are under hood and using a tube of some kind like sweeper wand to put to your ear and point at things you want to check for the squeal to verify exactly what/where it is coming from before you decide to fix anything. i just gave the most common answer to that type of noise. the possible tensioner cost of replacement will vary depending on who is doing it and the origin of the part. doing it yourself could be relatively inexpensive.