C# select value from textbox and multiply

Move the bracket, and don't do it like that! Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

DROP TABLE MyTable;

A perfectly valid "delete the table" command

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

As it stands, your query is bad SQL. If you manually expand it using "100" and "Hello" as the inputs, SQL is presented with a string like this:

Select pre_kod,ing_pav, pre_ska(* 100.0 /1000), san_kod from v_gaminiai where gam_kod='Hello'

That isn't right: Sql has to assume pre_ska is a function, and tries to work out what the heck the parameter is! It can't, and you get a error for the function and / or the parameter.
Compare with this:

SELECT pre_kod,ing_pav, pre_ska * (100.0 / 1000.0), san_kod FROM v_gaminiai WHERE gam_kod='Hello'

and it's much more understandable - but use double.TryParse to convert the textbox value (reporting problems to the user) instead of Convert.ToDouble and use parameterised queries to pass both values.