The short answer
Export contacts and deals from ActiveCampaign as CSV files and import them into HighLevel, bring email templates across with an ActiveCampaign share link, and rebuild landing pages, forms and automations by hand. HighLevel's ActiveCampaign migration guide covers each part; there is no account-level importer.
Contacts: clean the CSV before you import
The guide's ActiveCampaign steps are Contacts, then Export, choosing fields such as email, first and last name, phone, tags and custom fields, and naming the file without special characters such as ! # or $. Before importing, convert dates to YYYY-MM-DD, save the file with UTF-8 encoding, and delete line breaks and emojis. Create HighLevel custom fields for any ActiveCampaign field without a standard match. During the import, add a tag such as Imported from ActiveCampaign so the batch is easy to find, then spot-check a few records against the original.
Deals: build pipelines first, then import opportunities
Filter and export deals in ActiveCampaign, then check the CSV has a title, value, pipeline, stage, contact and owner for each deal. In HighLevel, create the pipelines and stages before importing, because pipeline and stage values must match ones that already exist. Import from Opportunities, map Deal Title to the opportunity name, and place each row in its pipeline and stage. HighLevel's opportunity import article adds that every opportunity must be tied to a contact, statuses must be Open, Won, Lost or Abandoned, and a finished import has no one-click undo, so run a small test file first.
Templates, pages, forms and automations
In ActiveCampaign, open Campaigns, Campaign Templates, hover over a template, click the gear icon, choose Share and copy the link. In HighLevel, create a new email template, choose the import option, paste the link, name it and create it; HighLevel's article notes the result is a code editor (HTML) template. Landing pages cannot be imported, so screenshot them, rebuild them under Sites, Funnels or Websites and re-add tracking codes. Forms are rebuilt under Sites, Forms with the same tags and actions. Each automation becomes a workflow with matching triggers, if/else conditions and waits, tested with a sample contact before it goes live.
More answers in HighLevel questions. For costs, see GoHighLevel pricing or run the cost calculator.