I'm sure this depends on where you live. Dexter semen is available from $25 per straw to $150 per straw. Your AI (Artificial Insemination) tech will charge you for performing the deed and perhaps a trip fee. It may or may not take on the first try, so factor this in as you make your plans. You may also need to pay your tech a storage fee for storing your straws and there will most likely be a shipping fee to get the semen.
On the plus side, you may be able to contribute to improving the breed by using quality semen over a local bull that has less desirable traits.
Hope this helps.