FTL liners and so forth travel at about one thousand times lightspeed, and the average nearest-neighbour distance between inhabited worlds is about 18 light-years, so the minimum interstellar trip is typically about six or seven days each way. For that reason rather than because of the cost, few people travel much, but some certainly travel at least a little. The inhabited volume is about 155 light-years in radius, so to travel from the middle of the Core to the edge of the Periphery and back is about eight weeks each way. The Imperial sector HQs are about 120 light-years (six weeks) from the Capital, and inhabited worlds are typically 20–60 light-years from their SHQs, median 48 LY (seventeen days).
These astrographical facts dictate that the terms of service with the Empire* have to be a bit like those of the India Office and the Colonial Office in the British Raj in the days of steamships and no telegraph. The Empire can't afford to have its officers waste a lot of time in transit, getting bored and fat in second-class staterooms. Perforce, a career with the Colonial Office consists of a series of four-year or five-year postings, each of which involves going to a world, working there for four of five years, and then being re-posted to the same place or posted away elsewhere. Casual trips home are not practical.
Terms of service include four weeks per year of annual leave, but they allow only limited accumulation. An Imperial servant must take at least two weeks of leave every year. That means that a typical pattern is to work for five years and then take a ten-week furlough — which is long enough to return home from a distant posting. Such furloughs may be extended with leave-without-pay. But not all Imperial servants choose to do that, or to continue it if they start. Many naturalise to Imperial society or to the planets they are posted to, and prefer to spend their furloughs visiting great Imperial centres, to the homeworlds of their ~spouses, or marvellous and attractive worlds near where they are posted.
The colonies are forbidden by the Treaty of Luna (~ the Imperial constitution) to terminate, suspend, diminish, or encumber the citizenship of any of their citizens who becomes an Imperial servant no matter how long they are away, or of their children. A member of the bush brigade always has the right to return to his or her homeworld as a citizen, without hindrance or let, either on furlough or upon retirement. (In point of law there are doubtless third and fourth-generation farmed fur who have an hereditary citizenship of some or several colonies under this provision.) But I suppose that only a minority do retire home. The minimum age for joining the Imperial Service is eighteen, and retirement age (except in the Marines) is 110, so it is very, very common for a career or series of careers in the Imperial Service to last more than 45 years. After 45 years I suppose that most people have forgotten their attachment to their homeworlds and birth families and formed new attachments either to the worlds where they have lived and worked or to the society of Imperial Direct Jurisdiction; they have formed new families of people they have ~married and children they have raised in those places. When they retire they often settle on the worlds where they have lived and worked, or on the worlds where their ~spouses (being younger than they) continue to work, or on their ~spouses' homeworlds, or in IDJ, or on some underdeveloped world where their pensions are princely, or on some world with idyllic conditions. And as a final consideration: some people join because they hate their homeworlds and despise their native societies.
So to answer briefly: yes, joining the Imperial Service is in effect an emigration. Not definitely and permanently, but about as certainly so as the emigration that my father undertook from the UK to Australia in 1948.
No-one will volunteer casually. Or if they do and get accepted, they won't go through with it. I reckon that all serious volunteers for the Imperial Service must either have a strong vocation to a kind of work that is not done or that they cannot get on their homeworlds, or they must have a strong impulse to get away from some aspect of their home. That might be poverty, as you suggest. Equally, it could be a stultifying social order, some sort of oppression or persecution, lack of prospects, or even an uncomfortable physical environment. I imagine that the most common thing that draws volunteers to the Imperial Service is the chance to do work that matters.
* Well, with the Colonial Office, anyway, or the Marines or Navy. In the Home Office you might get to serve in an Imperial habitat in your home system or a near neighbour. In Eichberger Spaceways you might work in a liner that served your home planet and get to visit home for a few days every month or so.
— Brett Evill
My SFRPG setting, Flat Black© My posts on SFRPG must not be reproduced beyond the board except with explicit permission from me.