Hi,
That is possible as well with HikaShop if you know your way around in PHP
To accomplish that, here is what I would do:
Create two characteristics: 1 for Trådfärg and 1 for the number of letters.
And fill them with the possible values
Then, create a product with those two characteristics. For each variant, set a specific price.
Then, edit the file show of the view product in Display->Views. There, add an input box for the text and hide the number of letters characteristics. Upon typing the letters in the input box, update in js the number of letters dropdown and trigger the js function to update the informations of the products with some javascript.
Finally, have a custom order field set in hikashop (business version) where you would be able to insert that text with some additional js and block the edition of it on the checkout. To force the value from the input text you added in the view to the custom field, you would have to edit the fields file of the view checkout to retrieve the value from the post and add it in the object passed to the code generating the custom fields.
The only limitation I see in HikaShop is that you can't have something which generates the price of an item as the price is always recalculated dynamically from the information in the database on each page (for taxes, zones, quantities, etc). So you need to have the price linked to a product in the database before being able to select it. So, for example, this page
www.almahouse.se/shop/7-sammetstyger/18-cerise-120-cm
where you can specify the number of meters. Either, you have a variant per possibility with a different price (and if you want it up to 200m then you will have 200 variants) or you need some code to generate the variants and the characteristic value dynamically when selected with some ajax. You could add in the view some code to generate dynamically the product variant and the price based on the options entered by the user. That would however require a bit more work as you might need some ajax for that. But that's not impossible either.